Q:

When is the true end of round (pre-flop, flop, turn, or river)?

What I would like to know is, on hold'em, lets say, 4 players:
P1. Check on 100
P2. Call on 100
P3. Rise 200
P4. Fold

My question is, after P4 folds, do P1 and P2 need to "Call/Rise/Fold" to match P3 (and then end that round), or the round ends with P4 therefore, P1 options would be "Check/Bet/Fold" to begin a new round.
Thank you!

A:

The only options P1 has are to call, raise, or fold. P1 could only check if he had already matched the highest bet (which he hasn't, because of the raise). P2 then has to call the highest bet, raise it, or fold, for the same reasons.
The fact that P4 folded has no impact on what options P1 and P2 have.
Edit based on comment: The round (pre-flop, flop, turn, or river) only ends when (1) only one player is left or (2) all remaining players have matched the highest total bet made during the round. So, if the example you gave was the play after the flop cards were dealt, then the flop round continues until all players call the highest bet or only one player remains. Only then does the river come out.

