Yes you are correct that the 4 is based upon seeing both the turn and river, however, the situation can arise where a subsequent bet is not made on the turn, so you get to see both by making/calling one bet.
Knowing exact odds for every situation is nearly impossible, in this particular hand, if you opponent held an under pair, such as 66, you would actually have 14 outs now, which would make you around a slight favorite post flop.
Conversely, if you held 6-7 in this position, and your opponent held QJ,, a 10 would be an "anti-out" giving you the 3rd nut str8 against the nut str8.
Aside from the mathematical aspect of actually making your hand, the other players could also fold to your post flop bet, however I would think that this would happen at a higher rate when you have position versus out of position.
Your table image will also increase the chances of a semi-
bluff causing other players to fold. If your a loose-hyper aggressive player, your going to get called down here by a decent player with good equity, possibly even re-raised.
If your playing a solid TAG game, the chances of your semi-bluff winning the pot outright are greater.
One of the keys of implied odds is how well hidden your hand is, an ace high flush hit it less likely to get additional bets called then a K-10 drawing with a Q95 on the board.
Knowing your opponent, what their ranges are, and what aggression level they play at, is as big a key in these situations as the math.
For the OP,,, here's a good video by Phil Hellmuth explaining the rule of 4 and 2.
I'll add one more to further elaborate.