I have not used the gen 4s so take this with a grain of salt but, I'd say 8, particularly if you don't want to run them very high over the tank. I have 3 gen 2 pros over a 180. Ultimately, I added 4 5 foot t-5s, 2 in front and 2 in back. I've had much better Acro success with this set-up. Regardless of lenses, I just think puck-style leds, act like 'point-sources' of light and you really want as many 'points' as possible evenly spread over the tank.
With just the 3 radions, I had lots of success with zoas, lps, montis, birds nest, etc., but struggled with many acropora.
One final thought- a par meter is your friend. I strongly suggest borrowing or buying one to help set up and tweak your lights. Your eyes (or least mine) are a very poor judge of intensity/spread. Using a par meter was a real eye opener (pun intended) regarding how uneven my lighting was with just 3 radions.
Matt