Need a board with greater than two layers which is open source

I'd like to practice some manufacturing techniques on a board with at least 4 layers. Can someone suggest some sort of Arduino board that has the Gerber files available which i could easily populate and test after i've built it. The function doesn't matter.

Thanks,
Kevin

This looks like it may be usable: OSH Park ~ Femtoduino USB, r14.2

If that isn't good, just search the shared projects for "4 layer" and pick something else.