If I cast Noxian Guillotine (without legion) on an opponent's 4 might unit that has Zhonya's Hourglass hidden, and then my 4 might unit attacks it in combat, does the opponent's unit survive or die?

The opponent's unit dies. Here's the sequence: (1) During the Combat Damage Step, both 4 might units deal damage to each other simultaneously. (2) An automatic Cleanup begins. (3) During Cleanup step, units with lethal damage are killed - but Zhonya's Hourglass replacement effect applies here, recalling the unit to base (healed) instead of it dying. (4) During Cleanup step (8), Noxian Guillotine's delayed trigger is finalized as a Pending Chain Item. (5) After Cleanup completes, the chain must resolve before the game can proceed. (6) Noxian Guillotine resolves and kills the unit. Zhonya's cannot save the unit a second time because it already used its 'next time a friendly unit would die' replacement effect during step(and Zhonya's kills itself when used). The key point is that the game does not allow a chain item to go on the chain and resolve without a cleanup occurring first, and Zhonya's only replaces one death.