It depends on the kingdom. If you have a cheap Village and Oracle, a $4 BoM could be crazy. Actually it's probably crazy with any $3 terminal and a Village on the board. And it's also very good in games with $3 trashing (or Chapel), especially if said trashing trashes from your hand. It would probably be good in Workshop/alt-VP rushes boards, too. And of course $4 BoM/$3 card is, edge cases aside, a strictly superior opening buy than a double $3. If you get to decide between playing a Woodcutter and a Chancellor, then it's pretty weak. Not getting to choose Throne Room, Remodel/Remake, Sea Hag or some other $4 situational cards is obviously a huge nerf, but they're not present in all BoM kingdoms anyway and decreasing the cost from $5 to $4 is a huge buff.
$6 BoMs would be too good.