I am not a good player, yet. But trying. Today while holding Q4 in BB I saw a flop of K4K rainbow. Original limper made small bet. Thinking my 4 might possibly be good I clicked odds to see what the odds would be in hitting my Q. The odds were not shown. While it is easy enough to figure in my head I was wondering if, based on those odds not being there, if I am just over thinking it. Is an over card in this situation of little or no value and that is why those odds not shown?
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In my opinion you are overthinking.
First, you already have two pair. Considering your opponent just limped, he likely doesn't have a high pair, so you are only behind if he has a king or a middle pair. Because this gives him trips or a better two pair than you have (or even a boat, but that is quite unlikely).
So the fact is, you likely are already ahead. Hitting your queen won't help if you are already ahead. Moreover, if he happens to have a king, hitting your queen still won't help because you will remain behind. Thus, hitting a queen only helps if he has a medium pair, because then your two pair goes from being worse to better. That's a small chance. No need for exact math.
To put it in your words, an overcard in this situation adds little to no value. Except it's not even really an overcard, since Q is under K. But in this case even if you had an A, it wouldn't matter much. The facts are about the same: 1) you probably are ahead in both cases, 2) if he has a K you are way behind in both cases, 3) if he has a mid-pair, then hitting your kicker saves you.
So, what to do? Since you are first, and you probably are ahead, you go ahead and bet. If he raises, fold, unless you know he's a wild guy. If he calls, it doesn't tell you much, because maybe he just doesn't think you have anything (and he should be right sometimes--you can make this bet even if you don't have anything).
That means if the turn is a blank, you probably have to bet again. If he calls again, try to check down the river. If he bets the river after you check, and you don't know him, you probably want to fold. But remember he did it, because if he does it often you'll have to start calling him.
For all of that...not much math involved.
Edit: Oh, I wrote for what you should do, being in the Big Blind, and therefore being first. You said he made a small bet, so I guess that means you checked to him on the flop. I don't think that's what you should have done, but since you did it...now you have to call. He made the bet with you showing weakness, so he thinks you have nothing. His bet does not mean strength, so you have to call it. Still no math needed. After that, pick up from what I said above, meaning when the turn comes, you take the lead and bet out.