He didn't say that it wasn't possible to beat the game w/ 4 BBs. I believe his point was that the game can get pretty freaking boring, since none of the BBs can use magic ever, and has to rely solely on HEAL potions to heal themselves. I'm assuming you've only tried that party with the NES version, since you use terms like 'lvl' and 'damage'. You really should try the four BB party on the PSX version. (On Normal mode, of course.)

The BB is an awesome character, but too much of a good thing goes sour, IMO.