Ok here is a question, Name some of the advantages of a trio band and the name the advantages of a foursome band..

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Mike

You don't have to split the money 4 ways in a trio at the end of a paying gig....

on the other hand...

You can't walk to the side of the stage where the hot groupie chick with the large tits is to coax her backstage after the show if you are stuck standing in front of the mic.... the bass player will get to do that instead...

you can sing and drink, you can play and drink, but you can't sing, play, and drink without sacrificing some of the attention that is needed to give the drinking the kind of attention it deserves...

You won't be able to bitch about how crappy the rhythm guitarist was unless you like talking to yourself...

Also on a serious note, unless the bass player is extremely kick ass, go with the 4 piece, otherwise it sounds like you are missing something if the bass can't play creative enough to make the lack of rhythm work...