What size of a tank would I need to keep 4-6 Angelfish (only fish in the tank) healthy?

Main issue you are going to have it territorial behaviour as they mature, rather than simple space to support them.

There are two good ways to keep Angels. In a compatible pair, or on a schooling situation in a large tank.

Therefore I would suggest maybe 75gal minimum, for a "school" of six. They aren't really a schooling fish, but a large group and plenty of space will defuse their territorial instincts and aggression.

Putting 4 in a smaller tank is likely to result in fighting when pairs form and they try and claim territory.

We have two angelfish, and their sizes are small that we can keep them in a small tank. They are very beautiful fish.

are you getting them from babies or adult?