For what its worth this doesn't necessarily always happen to 6 layers, just a thing that can happen. I have a 6 layer Wii that I've had since launch and its still chugging along and working great, just depends.
Also, yes, all colored Wii's are 4 layer boards, if you're getting one to use as a regular Wii, you'd probably want one with GameCube ports, as while you can restore that functionality on all Wiis, will require a shell swap, and that might not be very worth doing depending on your priorities.