$20/$40 high: routine fold

Obviously there is a reasonably good chance that he could have a pair of sevens with a big kicker, or a big three flush. There are a number of hands which play better heads up here, and, if he decides to play with them, he would be correct to raise with them, even if he is not ahead on third.

Having said all that, there are very few hands like that which are playable, and don't contain either an ace or a king. Given that it seems you are very likely to be behind, and there is a fair chance that you are badly behind, (kings up or better), then I think it is a good fold. Assuming that villain understands when he should play heads-up and when he would prefer multi-way, (thus significantly widening his hand range), then I think 4th is a call.