It's exactly that reason why I think combat squatting 4 psilencers in one unit isn't necesarily a bad thing.
First off, like you already pointed out... getting full use out of ALL those 4 incinerators would be very hard under 7th edition rules. The range of the template screws them over. Extra casualties from your cleansing flames won't help there either, like TompiQ pointed out.
Secondly, it's very unlikely that you can keep casting 4 cleansing flames every single turn. Aiming to cast 3 reliably is probably a better bet, especially with dice not being as endless a resource as they once were.
Thirdly, losing AP3 attacks isn't as much of an issue if you split them so that there's a more dedicated
CC squad and a more dedicated "shooty" squad afterwards. I think that may just be efficient. The alpha strike capacity of a psilencer is considerably less, but you can play them a lot more conservatively than just tossing everything withing 1-3" of your enemy.
I've not tested it out yet, but I think you'll be able to get more out of a "pseudo purgation" squad that way.