Vulcan-Centaur is the rocket that replaces Atlas V. The BE-4 is the engine on Vulcan-Centaur's booster stage, which replaces the RD-180 on the Atlas V booster

Vulcan-Centaur is the rocket that replaces Atlas V. The BE-4 is the engine on Vulcan-Centaur's booster stage, which replaces the RD-180 on the Atlas V booster stage. Both Atlas V and Vulcan use RL-10 engines on their main stage (what ULA calls Centaur). And to add more confusion, the new engine for Ariane 5/6 is the "Vulcain". :D