The 1/4" thick 4 X 4 is more than sufficient for your intended use.
Paint the top earth brown with a small can of latex paint and let dry or glue down a piece of I/O carpet.
Connect the track and lay it down on the board and screw down through the Fastrack holes with the screws mentioned above. You do not have to screw in every hole or piece of track in order to keep the track from moving around. I would start with a screw every other piece. Don't screw down too tight - just enough to seat the screw and keep the track from moving. If you are going to store it vertically, you may have to put a screw in every piece if it flexes when you tip it up.
If the screw heads protrude through to the other side, just cut them flush with a Dremel tool and cut off blade.
Dunskey.
