Note: If you want to use this data for designing your own PCBs, please ensure that my stackup data matches what OSH Park is currently using.

Here’s some data on the OSH Park 4 layer stackup:

If you want a 50 ohm trace on the top layer and the second layer is ground (as it should be) then you want the trace width to be 14.47 thousands of an inch:

I used Agilent LineCalc this time. It performs an actual e-mag simulation to synthesize the correct line width. I am quite confident that this is the correct value given the inputs.