Both level III+ and level IV body armor plates are designed to protect your vital organs from rifle rounds, however, there are some major differences in their capabilities, and design.
Level III+ body armor is an intermediate protection level between level III and level IV. It offers more protection than a level III plate, but less than a level IV plate. A level III+ plate can stop everything a level III armor plate can, but also offers protection against steel core penetrator intermediate rifle rounds, like the M855 green tip 5.56 NATO ammunition, however, it can not stop 30 caliber armor penetration or FMJ rounds like a level IV body armor plate.
Moreover, level III+ body armor plates are usually made in a single layer of either steel or ceramics, however, level IV body armor plates are usually made from two layers, which include a steel plate, with a ceramic strike face.
Level III+ body armor is also lighter than Level IV armor, and it is more affordable and comfortable as well. Level IV body armor is used by military personnel who are likely to face large caliber threats, like 30 caliber machine guns, and high caliber snipers. However, for a civilian, Level III body armor offers excellent protection against the most popular assault rifle calibers.