Multiple TVs on the same account

Yes and it depends on what you mean by "simultaneously".  You can have up to 20 devices linked to the same Roku account.  Depending on what you're watching you may be limited by how many devices of any kind that can access that provider.  For example, Netflix limits you to X number of simultaneous streams.  "X" depends on what level of service you're paying for.  Other providers have similar restrictions.  You can watch as many different providers (Netflix, Hulu, Paramount, etc.) as you want.  Also, it depends on your Internet connection how many simultaneous streams you can watch.  For 4 simultaneous devices all streaming at the same time you would probably want 100 Mbit/sec service.  The quality of the video makes a big difference.  Four simultaneous 4k videos takes a lot more bandwidth than four 1080p videos.