I've never heard it anywhere that a 4 sub setup somehow loses low frequencies. Not saying that you are wrong, but can you share your source?
I do know that some people complain that "I moved my sub out of my corner and put it on the middle of one of my walls right next to my seats, and there is less bass." Of course, there are lots of issues with that example. Corners make the bass seems louder and boomier, the sub placement impacts peaks (and nulls), so on and so forth.
Increasing from 1 sub to anything more than 1 sub helps to even out the whole low end frequency response. Maybe there are other factors playing into your example of 4 subs along the 4 wall mid-points (which to many experts is considered to be optimal for low frequency response) where low end is lost. Maybe it is just that enough of the nasty peaks (and nulls) have been lost that you are hearing less bass at certain frequencies and perceiving it as bad, when in fact it is just putting the bass at levels that they are supposed to be at?
Again, your source info would be great.
Oh, and I am by no means saying that you are wrong, just that this portion with 4 subs conflicts with what the general consensus among experts say.
Now, I will agree with you for sure that the difference in going from one sub, to 2 subs (even if not put into the ideal locations at the mid-points of opposite walls) yields the best "bang for your subwoofer buck" in taming the peaks and nulls and going beyond 2 definitely yields diminished returns for the price. It certainly does get better, but costs twice as much to get the last "20%" so to speak.
Here is a link to a YouTube video with Ian Colquhoun...
Multiple Subs 
