I think they're probably adding one station to their assembly line. That station adds an insulating coating to the external antenna (or some other mitigation of "the gap"), which will greatly reduce the problematic signal attenuation. That, and they get a grip on the white paint quality-control issues.
The white phones hit the market and show much less signal loss. Silently, the black phones get the same improvement. And the free case offer comes to an end.
People with original-issue black phones got a fantastic phone and a free case (if they wanted or needed one). In the white phone era, the free case is no longer available and the phone signal issue drops (no pun intended) off the radar.
Everybody wins.
People who bought original black iP4 feel cheated out of a phone that "just works" because Apple refused to do a recall.
Said people proceed to file a class-action lawsuit.
I guarantee it'll happen.