Don't know how well this will work for competition, but I have 2 different magazine holders from Uncle Mikes that I've used to carry 4 mags.
The first one is Uncle Mikes Compact holder. It has a snap AND velcro. The velcro is for when you need a quick close.
The second is Uncle Mikes Slim Line magazine holder. It is taller than the compact so it accepts longer mags. It doesn't have velcro.
Both are actually to carry 2 double stack mags, but when I was carrying a Kahr TP-9 I found both of them could easily carry 4 single stack mags.
What I really like about them is they take 1/2 the room on my duty belt than regular police magazine pouches.