This is normal with both the 231 and 242. The manual says that you can actually engage 4LO at speeds of up to 3mph, but since trying to judge 3mph is nearly impossible (and the sound of grinding gears in the transfer case is less than reassuring if you do get it wrong), the best thing to do is to stop with the transmission in neutral and engage it from there; it also really helps if you're already in one of the 4HI ranges before trying to go into 4LO. What I usually do is this:
- Come to a dead stop with the transfer case in either 4PT or 4FT.
- Put the gear lever in neutral.
- In one movement, shift the transfer case through its neutral position into 4LO. Do not pause in 'N' before going into 4LO.
This usually works. Sometimes it won't want to grab 4LO; if this happens, go back into 4PT/4FT, pop the gear lever back into drive or reverse, roll forward or back a foot or so, and try the procedure again. If it's feeling really obstinate you may need to try this a couple of times before you get engagement, but it's pretty rare for it to be that difficult.