The song in season 3 episode 4 when Deku starts up his 1000000% punch against Muscular
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