It depends on what turn it is, how the previous turns went and what cards you might've seen in previous games. (basically, how likely it is that they have removal, which is hard to judge given so little information).
The problem with playing prowler is that it could very easily not bait out any removal at all - they could just play a random unyielding krumar or salt road patrol and not worry too much about it. Or if it's later than turn 4, they could just play a larger green creature to trump the 4/3. There are just so many other ways to interact with a summit prowler that it's not always going to force a removal spell from them.
Inferring from the information you gave us, I'm guessing it's around turn 6, you've both played out 4 spells from hand each, and everything has traded off. In that case it really depends what their last turn was.
If they just played a land and passed, then you should definitely play the 4/3. There's 3 main categories of cards people can have in limited: creatures, combat tricks, and removal. Into an empty board, playing a creature is almost always better than holding up removal, so if they passed without doing anything, you would assume that they don't have any more creatures, which makes it more likely that they have removal. If their last turn was to play a creature, then trade it with your creature during combat this turn, I think I'd just play the butcher, as they haven't given any indication that they have removal, and butcher is the better play in a vacuum.
I'm guessing you played the butcher first, they had the removal, and then you regretted it later. The thing to realize is that in the cases where they don't have removal, the butcher is clearly the better play. And just because you played butcher first and they did have the removal, doesn't mean the play was wrong overall, it just didn't work out vs. their specific hand.