Ralf,

Early IVs (first ~200 serial #s) have fuel in the leading edges. You can tell by looking at the tip end of the wing. If the wire conduit penetrates the last rib mid span then you have a wet leading edge.

The leading edge also has a  joggle and the early manuals had you clamping the joggle with sheet metal screws that were later removed and filled. This was a  prime area for leaks. I helped one builder fix his wing by first draining, flushing and removing the wing and them drilling some inspection "portholes"  on the bottom of the wing ahead of the main spar between the ribs. I used a 2 1/2" hole saw on the outer skin and a 2" on the inner skin.  I then "sloshed" the leading edge with fuel sealer  using a mirror and a brush in a bent rod.  I made a remote LED light that I taped inside the bay for illumination. This was repeated for each of the bays. I then made closeout patches by putting release tape on the  wing next to the hole and applying a small lay up there. The patch then trimmed, fuel sealed and bonded onto the inner skin.  A foam core was then fitted with a margin so that the top bids could be "tucked" under the outer skin. A piece of 1mm plastic with release film was applied over the wet patch with tape to form the shape. I have attached a PDF with a sketch.

This method worked  perfectly and did not require skin removal. Total elapsed time was less than a week  and refinishing work was minimal.

Good luck.

Regards
Brent Regan