When you say 'drain flange', do you mean the quick drain on the bottom of the tank? If so, the stain should have been obvious around the flange of the drain. Leaking fuel will definitely migrate around during flight, but if you didn't see it around the drain flange, it's probably leaking elsewhere.
While you might be able to drill the rivets off the flange & use pulled rivets to replace it, you'll be leaving the rivet tails inside the tank. *Probably* not that big a deal, but....
Having pulled the tanks on a -4 several times, I can tell you that it's not that big a deal. There are no bolts through the spar, like the 7, 8, etc. If you're proficient with an impact driver (screw gun), you can have the tank off the wing in a half hour. Drain the tank (gotta do that, regardless). Remove the root fairing, disconnect the vent line, supply line, and float sensor wire. Remove the nose backet attach bolt. Start removing screws. Leave at least one screw partly threaded near the center on top, as a 'safety', but it's unlikely to be needed. If the builder dimpled the tank skins where they attach to the spar, you can slip thin plastic strips, or even popsicle sticks under the flange to lift the dimples out of the spar as you remove the tank (helps avoid scratching the spar). You can put a padded stack of 'stuff' under the tank for insurance. Once you're ready to pull the tank, remove the last screw. It's usually easier to get the tank 'moving' by pulling forward on the root end first. Wiggle it forward & off the wing.
Trust me; you'll drive yourself crazy trying to do the repair on the wing, and you'll likely miss (or cause) another leak in the process.
Charlie