Chasing Flush

Is it worth chasing a flush, if you have 4 cards to a flush on the flop

Depends....texture of board, odds (pot and implied), (near) nut flush or not.

There is not going to be one simple answer to this question as this is going to change from hand to hand, you really need to read up and understand how to figure out how many outs you have, how to calculate pot odds, both immediate and implied, understand discounted outs, etc. This is basic poker knowledge that you really can't live without or you'll be burning money. But speaking in a general sense, I think people tend to overvalue the amount of implied odds they have when chasing a flush as when the 3rd flush card hits the board it tends to kill any action, whereas with chasing a str8 people don't see it as easily on the board and you tend to get paid off more when you hit.

Not to minimize the other answers (which are completely valid) but just trying to give a bottom line quick answer (which I will admit can be misleading):

A big portion of your answer lies in these three items -

1) What kind of flush are you chasing? Are you holding a suited A? That's almost about the only time I will chase. Much less likely to chase if I'm not going after the nut flush or if I only hold one card of the suit in question.

2) How much is in the pot and how much do you have to call to see another card? What are the pot odds? Assuming you hold 2 suited cards and 2 of that same suit came out on the flop that means you have 9 outs to complete that flush. That means you have about a 35% chance to hit that flush within the next 2 cards, or it will happen for you 1 out of 3 times, OR 2 to 1. That pretty much means you can correctly call any bet that is anything less than or even up to pot sized to chase that flush. This just applies to the flop - do some homework to find your way after the turn.

3) But what if there is a pair on the board? That is a full house alert that you need to pay attention to because that will crush your flush. Be even more diligent if the bettor continues to give you odds with his betting.