A couple folks have hinted at this, but I want to provide additional clarity: the rule of 2/4 does not provide an Equity answer. It provides the probability that you will hit your outs. These are two different things.
If we have an Open Ended Straight Draw (OESD), we have 8 outs to hit our straight, or 16% chance per card. We do not have a 16% chance of winning the hand, which is generally how the term Equity is used. We may draw to the idiot end of the straight and loose to a better straight. We may loose to a flush. The point is to stop linking probability of hitting an out, to our equity. If we have a suited A, and are drawing to a flush, then we would have the nuts and our probability aligns better to our equity, as long as the last flush card doesn't pair the board giving our opponent a full house possibility.
We can only use the rule of 4 if we are guaranteed to see both the turn and the river  either because we will be allin, or our opponent is allin, or we are pot committed and will see both cards.
Semantics I know, but we need to make good decisions, and having a thorough understanding helps us do that.
Good Luck and God bless
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