The fourth world is a medium-technology world. Metalworking, electronic communications, skyscrapers and wide nature preserves. Technological entities and devices function as designed regardless of realm of origin, while magical entities and constructs will experience mana deprivation and its associated effects. Enchantments will persist as items are taken away from Spire, but there isn't sufficient mana to enchant new items. Magical creatures will get sick, though may be able to survive in a limited capacity.
The planet has one moon and orbits one star. There are four distinct seasons, with a wider range of temperatures than experienced in the first world. Snow is common in winter and heat waves are common in summer.
Records of the fourth world report it to be incredibly diverse in its biomes with a wide array of cultures. It's unknown how relevant or outdated this information may be. The third city is surrounded by a tall wall at roughly the same distance from the Spire as the outer boundaries of the second and fifth cities. Outside the city wall is a ring approximately two miles thick, a flat plane with sparse grasses. Beyond that is the outer wall, which was built and maintained by the human population of the fourth world.
Every entity that has attempted to cross the plane from Spire has been shot and killed by outside human snipers before they reached the half-way point. No known attempts have been made by outside humans to enter Spire since the uneasy end of the Convergent War, almost two centuries ago. What we know of the fourth world is reported by the human population of Spire and may have changed in any number of ways since contact was cut.
The fourth world has one sapient species, the humans. They are mammalian, averaging between 5' and 6 1/2' tall, and have relatively prominent sexual dimorphism compared to the other species of Spire, with only an estimated 3% of the population displaying innate intersex anatomy (though this is difficult to determine in the majority of the population, as will become clear as their magic is explored). Pre-convergence humans had a relatively rigid binary gender system, though the culture and norms of Spire humans have changed more than any other race in the centuries since convergence, likely owed in part to their lack of contact with their parent culture. Modern Spire humans vary wildly in the importance they place on gender, sex, relationship models, and even child rearing structures. They give live birth and rarely bear more than one child at a time.
The third world has no natural mana sources outside of Spire. Until convergence they had no knowledge of magical systems and only enough scant mana leaking in through the bedrock of Spire to power the first spells which brought the fourth world into the convergence. The very existence of this bedrock though, and the ruins which formed the beginnings of the third city, imply a much older and mysterious relationship with magic which modern human historians are eager to uncover. Because of their lack of native mana flow human society has advanced significantly in the field of metalworking, with many of their technological advancements revolving around the practice. Iron, in both worked and raw forms, was commonplace.
Third world humans who perform no magic fall strikingly close to the theorized "average entity" (a controversial concept even within exo-anthropological circles). They appear and behave quite similarly to many, if not most, of the societies an exo-explorer may find on their travels. When they perform magic, however, their bodies are affected in permanent ways. The mutations are typically minor and thematic to the nature of the spells cast, but they accrue and accumulate. Many adult humans appear, to an ignorant observer, to be of different species entirely. A human mage who specializes in fine detailed crafting may eventually find themselves shrunken in size, with extra limbs and exaggerated large eyes. Another who specializes in botany and gardening may grow leaves from their skin, grow tall and willowy, and find their sense of time slowed. The intensity of mutation is proportionate to the intensity, frequency, and consistency of the spells cast, with systemic discoloration being the most common.
Human magic is performed by speaking words in a particular ancient language. Any declarative statement made in this language becomes true if there's enough mana available in the area to realize it. It is a theoretically world changing ability, a language of gods. Theoretically only, as modern humans have deciphered only a small sliver of the language's basic vocabulary and grammatical structure. The fourth city is built atop a set of ancient ruins discovered in the decade before convergence. The ruins are covered in carved script, and several small libraries were uncovered, all of which have long since been meticulously cataloged. Much of the study and advancement of human magic revolves around puzzling out the proper phonetic pronunciation of the ancient words, and the definitions of various words based on the effects of those spells which contain them. In the last century, as more of the surviving vocabulary is understood, focus has shifted increasingly onto grammatical structures and conjugations in an attempt to deduce further unrecorded vocabulary.
This language produces no discernible effects when spoken by non-humans.
The language of magic is not the language of everyday communication for humans. Indeed, there is very little magical vocabulary that is universally known among the population. Some spells are widely used, simple things, but most are relatively unique to different hobbies, occupations, and studies. The grammatical structure of human spoken communication falls somewhere between that of their magical language and that of the common, signed, language of Spire.
The culture of Spire humans resembles, in many ways, the disparate cultures of the courts. They are an outcast people, the descendants of those researchers who chose to align themselves with the city when the ancient outer humans declared war on the place. Like the courts, they lack an external society to tie them to old ways of being, and have adapted heavily to life within the city. Unlike the courts, they are saturated and real, and have far fewer limitations in how they're capable of interacting with the world, other sapients, or each other. Still, there are some unique aspects to Spire human culture. They value education highly, and have a strong academic culture. Knowledge, study, diligence, and specificity are valued. Humans often find themselves in conflict with fae who are inherently vague and imprecise in their language: a human likes to know exactly where they stand and exactly what is at stake.
Their family units are more variable than those of other races, as different relationship styles come in and out of fashion across the generations, but 2 parent or multigenerational homes are always common. In the last century there's been a rising trend of tightly knit peer groups who live together long term, composed primarily of nominally monogamous pairs. The close family unit, then, consists of two parents and their children, but many of the responsibilities of raising children and maintaining a home are divided fluidly between members of the group.
Human religious practices vary widely and are considered very intimate and personal. They draw heavily from the cosmologies and practices of the other worlds, but tend to include stronger elements of fate and destiny, fortune telling, and luck. Small household shrines or altars are common. Sometimes these are dedicated to deities or personified forces, but more typically they are representative of concepts or virtues which the humans value and wish to embody or manifest in their lives.
Humans are omnivores, and cuisine is a valued art form in their society. As there's a lot of overlap between their diets and foods which the other local sapients can digest and enjoy, human restaurants often offer a variety of dishes for anyone who may visit. Most of their holidays feature food and feasts as a centerpiece of the celebration.
The human government consists of a council of elected officials, each of which supervise an aspect of the city in which they are experts. Although any human can attempt to get elected, it's exceedingly rare for any to actually gain the required votes without first putting in years of study, research, and training in the appropriate field. The council members select a representative to serve on the Spire parlament, usually an individual skilled in leadership and negotiation. This representative delegates assignments and research questions to the council members and holds a lot of executive control over the fourth city, but can be replaced at any point by a unanimous vote from the council members.
Humans are generally considered knowledgeable, bureaucratic, superstitious, and driven. They are extremely valuable assets within their areas of expertise.