We have 60Mbps from Time Warner/Spectrum. Me, my wife and three kids have had as many as seven sources of streaming video simultaneously. I had three ballgames, and everyone else was watching something (Netflix, Amazon, Youtube...).
Now if you want 4K content, you might max out a 60Mbps pipe with two or three 4K streams.
If all four of your TVs will just get streaming content, then simply chose a device for each of them (Roku, Amazon Fire, Apple TV...) and plug it in. Make sure they've got a good wifi source, or better yet wire them directly if you can (not an option for most people).
If you want OTA channels (which I high recommend since they're free content and have 50% of the most popular shows) then the easiest thing is to simple buy an indoor antenna for each and plug them in. This however might not be good for some TVs that can't get a good OTA signal, such as a basement TV. This is where some people put an antenna in the attic, or on the roof, and wire it down to the TVs, perhaps even splitting it for multiple TVs. This is more of an art than a science to get a good signal to TVs in bad OTA locations. An easier way might be to do what Otis did and buy a Tablo. Put one antenna where it gets great reception and then use a short cable to plug it into the Tablo. Connect the Tablo to your wifi, and all your devices just use the Tablo app to get OTA content.