Hey,
I think you should lead or check/fold on flop.
Leading works as a reasonable bluff with equity and usually helps you realize your equity as well. On 4 to a straight you don't have implied odds or a disguised hand to check/call with pot odds + implied odds. That's why if you check it is to check/fold. When you lead though, you still have equity and you have good FE. Your opponents will usually just fold AK, Tx, 9x and lower pairs. This doesn't always happen of course, so you have to pay attention to CO since he is the one with a strong range and short stack.
MP is bound to have many bad hands preflop so against him you have the required folding equity, while if you get raised it's a very easy fold. The safe play is fold flop when you got into this spot.
That said, I would like to comment on preflop as well. I believe calling here is a very big mistake. What's CO's 3bet? What are his postflop tendencies? In 14k hands you can get to know his game better than himself! When you are OOP with such a weak ace facing a 3bet I think folding is the better play. Not only is CO likely to play correctly against you but MP is still left to act and can 4bet. You never have a hand that can face a 4bet when you flat here or simply aren't perceived to have one.