You only ever multiply your outs by 4 on the flop if there will be no further betting. In other words when you will be all-in with your call or bet. This is because if you are all-in you won't need to re-evaluate your

odds on the turn.

If betting proceeds normally however you multiply your outs by 2 on the flop and then again by 2 on the turn. This is because the reason we're calculating our outs is to see if we're getting the correct

pot odds to make a call.

Here's an example. You have AcKc and there are 2 low clubs and another random low card on the flop. The pot is 2K and in order to play you must call with your last 500 chips which will put you all-in. So we say you have 9 outs to make the flush plus 6 outs to pair either the Ace or King which gives us a total of 15 outs. Knowing that we'll be all-in if we make the call we can calculate that we have a 60% chance (15 x 4) to win the hand by the time the rive card is dealt. We're getting 4 to 1 or 25% pot odds to call. If we subtract the pot odds from our winning odds we get 35%. This means if we always make this call here then we make money in the long run.

However, if you had 10K chips instead and you must call for 500 to win a 2K pot the picture changes. We still have 15 outs but because we will have another betting round on the turn we cannot calculate up to the river. We can only calculate up to the turn. So 15 outs x 2 is 30% chance to make our hand by the turn. The pot odds is still 25% so if we again subtract our pot odds from our win odds we get 5%. So we'll still make money in the long run if we make this call but not as much because we will likely have to put more money into the pot on the turn to see the river card. If we miss on the turn we must again do the math based on whatever bet our opponent makes.

And this is why we only multiply by 4 if we are all in. Because we cannot know what bets will be made on the turn if we are not all-in. And if we are all-in we don't care about that anymore.