I actually did this last year. And not just four molds. Except for five rounds at courses with a high potential for losing a disc, I played all of 2011 using just four discs. I play a lot too - it was easily 400 rounds.
My four discs were a JK Aviar, DX Roc, DX Eagle-X, and DX Valkyrie. Same Roc and Aviar all year, but I went through about 3 Eagles and maybe a half dozen Valks.
My original motivation was for the fun factor. I'm an old guy and played for years using a single disc. I wanted to get back to the feeling of freedom of those single disc days but I wanted to stay competitive in the local scene.
It worked out better than I'd ever imagined. I did have more fun ditching the bag o' plastic just as I'd expected, but I was surprised to find myself playing the best disc of my life. I believe the reduced decision making improved my focus and I think I gained a familiarity with the discs I use that you guys using a bag full of different molds will never know.
So this was an experiment I committed to for 2011. It's 2012 now and I'm trying to evaluate whether I really gain any advantage using a 10 disc bag. I'm not really seeing an advantage as far as my game goes, but carrying a 40 of beer is a lot easier using a bag.
I've seen people posting that "it's the Indian, not the arrow", but if they really believe that, why do they need that bag full of plastic? But hey - whatever makes it fun for you. If it's just a security blanket, like a lot of my friends have told me, be brave and try your own four disc experiment. I think you'd be surprised.
Anyone looking to discuss extreme disc minimilism is welcome to visit a little Social Group I started here - I carry my discs in my hand