*Spoiler Below!*
Not too long after unlocking the second island you'll come across a mission that involves saving Lance after he was captured. Once you complete this mission, you may want to make a save file as a backup in case something goes wrong.
When you start this mission you'll be given the M4 from the start, if you want to play it safe make your way through the mansion using a different weapon so you don't run out of ammo. Once you reach Diaz, simply use the M4 to take him out. If you're having trouble controller the recoil, you can actually fire this weapon without using L2 to aim. Holding R2 alone will have Tommy fire it without recoil or first person.