Re-introduing them will not help after 4 years. You need to do some serious training with your female. (You didn't tell us her name!). I'll just call her "Girl" for now.

Girl thinks that she is in charge, and so tries to assert herself by bullying Radar. This is wrong. You need to do some serious training with her, apart from him, and let her know that YOU are that alpha dog, not her.

Several people here have written on this in the past.

How old are both dogs? How big? Does one get more attention than the other? Where do they sleep? Are they fed together?

Yes, that's a lot of questions, but we cannot really help until we know answers!