you can only get 3 in a single run. the fourth you will get when you finish the game and unlock the extra missions in the training room.
Four enemy skills are as follows.
Self-Destruct - Learned from Smogger - These guys are everywhere just beat them down and stand beside there body to learn the move. Should get this one easy. If having Issues finding one look on (Chapter 14) Sector 5 Slums Area. If you haven't learned this by Chapter 16 you can use the training room to fight these guys and learn the spell.
Algid Aura - Learned from Cerulean Drake If you haven't learned this by Chapter 16 you can use the training room to fight these guys and learn the spell.
Spirit Siphon - Learned from Phantom - If you haven't learned this by Chapter 16 you can use the training room to fight these guys and learn the spell.
Bad Breath - Learned from Malboro - Chapter 16 Only in the training room. Can only be done on hard mode.