[mGGPrometheus]

4gate should be quite easy to spot. Send your barracks SCV to scout. If you see that he's saving energy on his nexus, it's likely he will 4gate. If you see him chronoboosting his cybercore, that's a dead give-away for a 4gate.

If you're doing the standard 1 Rax FE and you spot a 4gate, you should throw up 2 bunkers. If you have been continuously pumping with your 3 barracks, you should be able to hold your front until stim/medivacs arrive, at which point you will steamroll the protoss, since 4gate is an all-in timing.

4gate is one of my favorite builds to fight against, since early game protoss aggression is quite easy to deal with compared to a late game deathball.