The song in season 3 episode 4 when Deku starts up his 1000000% punch against Muscular In season 3, episode 4 of My Hero Academia, the song starts playing at around 20:22 when Deku starts up his 1,000,000% punch against Muscular.

I've been searching around but I can't find it anywhere.

What's the name of the song?
The song you're looking for is from the soundtrack of Boku no Hero Academia season 2 OST, by Yuuki Hayashi.

The name of the track is Koutekishu to Kaite 'Tomo' to Yomu.
It is the 14th track on Disc 1 of the OST.

My Hero Academia 2nd Original Soundtrack