Re: Population limit to unlock 5th area

It is possible, the trick is to have a solid number of residences built in your regions, and getting as close to having a 100% population boost coverage from each building specailty type. Helioport (transportation), Snake Eye Casino (Gambling), University (Education), Ferris Wheel (Entertainment), and the Giralada (Landmark) were my favorite buildings to use for boosting population and those buildings are cost effective as most of them are low cost in terms of using gold keys / platinum keys.

Landscaping around and in between the buildings with parks (the small ones preferably to save space) also help boost population.

In terms of what kinds of residential buildings to build, I recommend the regional residential buildings, Tokyo residences, regular residential skyscrapers that become epic buildings, and Omega residences are the best building in my opinion as these building types hold more population than the rest, and match that with specialty buildings, landscaping, and parks, and you can hit 10,000,000.

You just have to really work for it.