As mentioned by the other guys, it should reject the command as shifting to 4L would require the tranny in neutral. However, I'm not sure what it will revert back to. I would imagine it would revert back to the previous setting (so if you did it from normal, then should go back to normal). It will engage 4A and 4H at speed though. I believe it is up to 55 mph. I like experiments; I might try it and see. When in doubt, #RTFM.