The Renli's have 2 electronic parts to the 4wd system, one is a little black box that sends the signal to the electric motor on the diff. When there is a malfunction with the 4wd the box "beeps" to let you know there is something wrong.
I replaced the box that send the signal, that didn't solve my issue. It is most likely the electric motor on the diff, when I put it in 4wd the motor doesn't do anything or make any sounds so I'm assuming it's dead or only works sometimes.