Appropriate infrastructure for one scale usually isn't appropriate when the population grows 4-5x.
A cluster of 4 1000-pop villages will usually share an elementary school, with students bussed 10-20 miles to it. A 4000-pop light suburban town will have its own elementary school, usually one where parents will drive their kids 1-2 miles to it. A 4000-pop dense suburban development will place a 1-2 story elementary school and associated park & fields inside the middle of the neighborhood, where most students can walk to it via footpaths without crossing major roads. A 4000-pop urban neighborhood will build a 4-6 story schoolhouse on a major avenue where all the students are close by.
A cluster of villages or small towns will have 2-lane roads snaking between major village centers. A dense suburban neighborhood has a road hierarchy, 6-lane boulevards with commercial strip malls, fed by 2-lane residential streets. An urban neighborhood builds mass transit.
In each case, you can't really upgrade the existing infrastructure to the point where it's appropriate for the new density. You have to tear it down and build it a different way, at a cost that's usually higher and more disruptive than just building it a different way in the first place.
It makes me wonder why we don't have more bands of underhoused Millenials going off into the wilderness to found new cities. That's how we got the infrastructure in the first place: land developers generally subdivided undeveloped farm/ranch/wilderness land, and sold it to people living in overcrowded tenements in the city. Or in the case of Europe/Japan/Russia/China, bombed the hell out of each other until all the infrastructure was destroyed and then built new infrastructure from scratch to house the survivors. Perhaps a tongue-in-cheek-yet-actually-practical way of looking at this might be to treat cities as disposable, with a lifetime of a few generations, and then once the people outgrow them it's time to abandon the city and build a new one.