I have the exact same problem with my X button. All of the other buttons work perfect, it's just the one button that keeps sticking. It's just the one side of it that gets sticky too, I figured it has something to do with the way I hold the controller when I play. I'm glad to hear I'm not the...
I believe the pc4000 will just step down to pc3200 speeds when you put them in together. You aren't going to damage anything by giving it a try anyway.