Level 3A vs 3 vs 4 Body Armor | Guide to Body Armor Levels
TL;DR
If you want to stop standard handgun bullets: Level 3A body armor
If you want to stop assault rifle bullets: Level 3 body armor
If you want to stop high caliber armor penetrating bullets: Level 4 body armor
Note: Atomic Defense Level 3A body armor also protects against 12 gauge slugs and buckshot