If you have a hand like 44 and flop an underpair to the board or maybe third pair, usually the best play is to check and fold to any significant aggression. This is why, we need good implied
odds to setmine. And if we are not getting these implied odds, we should just fold preflop. Its a common mistake to believe, that the smallest pairs are strong
hands, that must always be played. This is absolutely not the case, because they are almost never significantly ahead of anything, and they have very bad
equity realisation postflop, except for implied odds when flopping a set.
If stacks are short enough, we can sometimes move all-in preflop with a small pair though, since this takes away the problem of having to play postflop. Lets say we face a min-raise from CO, and we are in BB with 44 and an 18 BB stack. Then the best play is to move all-in, because in that way we can often make our opponent fold. And if they do call with a hand like AJ, then we win their entire stack, if the board runs out something like KT8-2-9. Whereas had we just called and seen a flop, it would have been very difficult for us to call their C-bet, so we would likely have ended up folding the best hand.