Up until this point your decision making process has been relatively easy on whether to proceed or muck your hand, but on fourth street the action will pick up and define how many players actually may have improved their hands. If their fourth card shows an overcard to your high pair, or a card that may improve their flush or straight draw, you must consider what you will need to beat your opponent and as always, consider how live your cards remain. If you still believe your high pair is still the best hand, continue to push it again. Your bet is an attempt to further eliminate players and if anyone still remains in the hand with a smaller pair, you have achieved your goal. Note: If your pair is not the highest pair, and you have overcards that are higher than your opponent's pair, then continuing to play is a correct decision.
Some quick tips:
Playing two pair is the hand that can cause you the most difficulty and grief if not played correctly and playing middle value hands correctly can make the difference in a winning or losing session. Anyone can play the huge hands correctly, so it's imperative you also acquire the skills to play these medium value hands with equally adept maneuverability and aggressiveness. If you have two pair on fourth-street and no one is showing a pair higher than yours, you must bet out to protect your hand. You must reduce the number of competitors thereby improving your chance of winning the hand. If you bet and everyone folds, just scoop the pot and be glad they held up.
Here are some other important things to remember about playing two pair on fourth-street: The key remains; you want to have live cards and overpairs. You need to have absolutely as many live cards as possible to improve or your hand will not hold up and others will outdraw you by the river. However, even if you don't improve, holding an overpair will enable you to win more pots if your opponent also ends with two pair. You should also be aware that while playing at medium and low limit tables, you should not limp with two pair on fourth-street or you will not enjoy the outcome on seventh-street. You must protect your hand, and play your two pair hard. Having said all of the above about playing two pair, it should become abundantly clear that you would not want to be in many hands holding two small pair. These hands do not hold up to here and a person holding one higher pair on fourth-street is likely to wind up with two higher pairs than your two pair. Avoid these hands if at all possible.
When you start with a medium or small pair, you must always proceed with caution if your hand does not improve and you perceive your opponent to be on a bigger pair. In most cases you should not proceed and definitely not if the pot is raised. The only exception on fourth-street is when your cards are live and three of your four cards are of the same suit. In this case it may be correct to call an unraised pot.
If you start with a drawing hand (three to a straight or to a flush), and you did not improve on fourth-street, you can only continue if you have one or two overcards. If the pot is raised, don't bother to call. If you are lucky enough on fourth-street to get a fourth flush card or fourth card in succession for your straight, you still have to keep an eye on any opponent with a pair showing. You are in excellent position to complete your drawing hand, but your opponent has a decent chance of making a full house. If you start with hands like T-J-Q and miss your card on fourth-street for the straight and miss pairing any card, you should consider folding unless given a free card to continue. Should you catch an inside straight draw after receiving your next card, use some discipline in deciding whether to continue drawing and paying a heavier price to play. If raised, abandon these inside draws for certain.
Lastly, continue to watch your opponents, attempt a semi-bluff or steal (by betting or raising) if you have position and think fourth-street cards did not help the limited amount of opponents remaining in the hand. You don't have to have the best hand here, it may just look like the best hand to everyone left in the hand.