How high is the ceiling? That will make a big difference in which/how many speakers you need. A high ceiling means you can hang speakers with narrow dispersion higher (which requires more power) to maximize coverage OR use more speakers with wider dispersion. A low ceiling means you need wide dispersion speakers and there's no option for hanging them higher. Assuming the walls aren't covered with soft cloth or absorptive materials, you need to consider the floor and ceiling materials when you determine the speakers and power output. Hard floor and ceiling means you can use the reflections to your advantage, but you won't be assured of great sound unless you do some serious signal processing, especially when you throw a mic into the system. Even with an impact-absorbing floor material, feedback is almost assured, so some kind of feedback killer will be needed.
Decent commercial amplifiers and small mixers aren't terribly expensive, so a receiver isn't needed. Whether you need 70V- I would say "Maybe". The room is 4000ft² but knowing the room's volume would be better. If the ceiling is 10', you have 40,000 ft³, which is a decent size but the average and maximum SPL will be a consideration. I have a speaker configuration program from JBL, if you want to let me know the SPL, ceiling height, etc and it's designed for best coverage for the area.