Updated: Dec 28, 2022
An NIJ Certified Level 4 armor panel, will confidently in all situations, stop M855A1. M855A1 is NOT an armor piercing round. It is what we call a barrier blind round. A projectile that is meant to maintain good ballistic properties even after penetrating light barriers such as plywood or drywall.
The steel tip, does not extend all the way through the projectile, that's just a penetrating tip for barriers. The core itself is actually a softer copper. As you can see in the side view of the M855A1 round on the right.
Level 4 is tested/ certified against .30-06 M2 AP, which is a 162gr armor piercing projectile with a hardened steel core, traveling at approximately 2850 ft/s. Its an incredible amount of energy and power and it far dwarfs the M855A1 round. And yes, level 4 can stop the standard M855 as well.