Animation Blueprints are by themselves not that expansive. They also have the ability to be set to run in parallel and they have indicators when you are doing something a little bit more epansive. They show the easy fast calculations with a flash icon to indicate fast path support.
Using it with 4 actors is absolutely fine. You run into problems when you need thousends of them. There are different solutions to handle those cases.
- You use level of details in the animation blueprint itself
- You swap out the close ones with actors with animation blueprints (Done in the epic city sample)
- You do some animation sharing between actors (Epic used this for a zombie event in fortnight)