Well, on a fuel injected system, removing the gas cap may not relieve the pressure.
Look under the hood on the fuel rail and you will find a schrader valve. Put a rag over the opening and push the valve nipple to release the pressure. You'll hear a small pssshht and a little gas may come out. Often if the car sits long enough, the pressure will bleed off on its own.
It sounds suspicious not being able to remove the fuel line nuts though, you may want to double check that you're turning the right stuff!. You should have a fuel resistant catch pan handy as gravity will let whatever is in the lines (and tank) flow out.
Note: The schrader valve advice above is generic and may not directly apply to your car. Mine is still too new to need a filter change, so I don't have 4Runner specific advice.