This probably should be in dumb questions section and it probably sounds utterly retarded but in my head it makes sense, fairly confused about it

Say you 100% know your behind in a 3 way pot (this is from a hand a played recently) and your last to act but you have outs (10 if my reads are right up against a set or 2 pair) and im facing a http://www.eonlinecasino.org/free-blackjack
raise and a 3bet minraise, i'm getting 3:1 when action is on me so by my calculations i'm getting the right odds?

Now i'm am sure the first raiser is gonna ship it, im not worried about the 3better he was doing this with 2nd pairs and top pair(which i beat) all day, so the first raiser ships it and the fish calls.....now i am getting 4:1 and call

My question is what is the difference in 4bet shipping it when i know im behind and i'm getting called by a better hand, and only calling the 3 bet and thus creating the right odds for me to call making it profitable

I don't have a hh for the hand