If you have 4 Might unit and Rengar Legend, if you play Kinkou Initiate does the trigger go on the stack if you don't already meet condition when you play trying to buff your 4 Might unit to 5 with Rengar?

Yes, the trigger goes on the chain. The "if your other units have total Might 5 or more" on Kinkou Initiate is part of the effect, not a condition that blocks the trigger from being placed on the chain.

Both triggers — Kinkou Initiate's "When you play me" and Rengar's Pridestalker "When you play a unit, give a unit +1 Might" — trigger simultaneously when Kinkou Initiate enters. You, as controller, order them on the chain (Rule 333.1). Put Kinkou's ability on first, then Rengar's on top. Rengar's resolves first, buffing your 4-Might unit to 5, then Kinkou's resolves and sees 5+ total Might from your other units — so you draw. This is confirmed across multiple FAQs (#9873, #10875, #11572).