90% of the time they are interchangeable......generally speaking, I look for more speed, bh and pass out of my 4 than 5
In a 2-3 zone, the 5 really needs to have a high BLK value.
In the press, I like to have more speed (say 40) in the 4 but will live with a center in the teens.
Normally, I just try to move my better defender to my opponents better offensive option.