Not sure if it is the same on the 4runner but all my 8+ year old subaru's use to have the gas smell when it was cold. The rubber fuel lines get hard with age and then would let a little bit of fuel leak (not enough to see) and then vaporize and smell. The first few years I just gave a tighten to the clamp but that only worked so long. Replacing the lines solved the issue (or at least till I sold the car). I'd suggest sniffing out the source as grecko suggested but look for any rubber to metal connections as a possible source also.
