Try to sign coaches who have 5 star attributes in 2 areas so you can mix and match when there ratings drop. Unfortunately with goalkeeping coach you are stuck. To be honest I haven't noticed much if any difference between 4 and 5 stars when it is coach who was originally 5 and dropped to 4. I went a whole season just playing games and when I checked my training the shooting and ball control had dropped to 4 stars. I don't know at what point this happened but it didn't have any noticeable effect on players attributes i.e. no downward red arrows.
The thing is that coaches attributes are treated exactly like player attributes in that the 1 to 20 attributes and associated star ratings are actually represented by a larger number in the game engine. I don't know what exactly this value is but as an example assume it is between 0 and 200. Hence 1 point on the attributes you see actually represents 10 points in the game engine.
e.g.
Coach A: Game engine Attacking = 190
Attacking attribute you see = 20
Coach B: Game engine Attacking = 197
Attacking attribute you see = 20
The point I'm trying to make with this rambling description is that if your coach was 5 stars and performing well, and then drops to 4 stars it is propably because he is on the cusp between the value brackets and thus it won't affect greatly how well he performs.
So if Coach A above drops from 190 to 189 you will see that as a drop from 20 to 19 and his stars may drop from 5 to 4, but it will have negligible effect on how he performs.
That's my 2 cents and I could be completely wrong