More than a team of four can manage certainly.
It does not, however, take a constant number of engineers to maintain the infrastructure that lets a single engineer push a feature to millions of users. (P/I)AAS can help, but you still need people to monitor performance, find regressions and bugs, and track them down.
Uber only works if there are enough drivers so that I can always get a ride. If I get told that I have to wait 30 minutes for a ride more than a tiny handful of times I'm uninstalling the app. Growing organically in region would be very tough since it would take too long to get enough drivers in place that users could rely on the service and conversely drivers would be unwilling to sign up because not enough users where bothering to use the service.
Those 4 can handle everything backend related too?