In a 100BB spot where the 3-bettor usually folds or jams, the 4-bettor has a polarized range; a hand he'll fold, or call.

The folding hands are usually still strong starting hands, but they're not good enough to get it in with. Most players tend to use hands that block value combinations from the 3-bettor.

KQ for instance blocks AK/AQ/QQ/KK and even his pre-flop Kx/Qx bluffs, so the 3-bettor has fewer hands to jam with.