depends on when you are planning on using it... launching in 4x4 with stock shafts and then locking it as soon as you're moving, yes, it's bad... you won't have input/output shafts for long.
Locking it once it's in 4th (it should be locked on it's own anyway) and using it to help you slow down going down hill or something, or when using an exhaust brake, as long as you turn it off before it needs to downshift to 3rd (it won't downshift from 4th once you lock the switch unless you unlock it), it shouldn't hurt anything.
Use at own risk, no gurantees, warranties, or other implications... things can/do/will break... . etc... .