I realized that I made another potential mistake with the countersinking. The 9 holes in the center of the front spar do not have skin over them so they really should have been countersink flush to the spar flange. Typical practice to accommodate a dimpled skin is to set the countersink .007” below flush. Of course my cage was set that way and the resulting rivet heads sit below the surface of the spar flange.
This would be merely an aesthetic annoyance, except I can't squeeze them tight down in the recessed hole, the rivet will essentially sit proud in the hole and not make a strong joint.
I considers oversizing the rivets but there is not adequate edge distance for a -4. My other idea was to go ahead and put a narrow strip of 0.025” dimpled “skin” on top of the spar.
I talked to Van's and they agreed that would be acceptable.