I've had the same problem with trying to go from size 2-3. Even if i have the money I can't pay for the breast enlargement til I fail the $20 paid experiment. After I fail Then I can spend $50 to go from 2 to 3, but will only ever reach 4 after the farm in its current state. The "Dream" Bimbo sequence has been the only way I've managed to get to breast level 5 so far.
Now In a few day's I'm going to try to Reduce The Breast size to level 1. then see if I can pay my way to level 3, then use the $20 experiment one to go to level 4, and final 5 at the farm.
Breast size 4 could be locked to the bimbo route or a bug. I haven't played a previous version to know if there was a difference.