REMEMBER what I found out the hard way when the bigredbass lost the AXY's for new FatBoys:
I see you want to trade FatBoy FOURS for AXY FIVE/SIX's: If these are for the same bass, fours and five/sixes are DIFFERENT shell sizes, and won't interchange without surgery.
'Classic' taper fives use four-string shells as I found out ! !
J o e y