The shifter cannot literally be shifted into 4wd, 4-low or 4-hi, or it can be shifted but isn't engaging?
Shifting should be made using quick motions. Shifting slowly may make it more difficult to shift.
Have you tried realigning the gear teeth in the transfer case?
If the transfer case shifter is in the Neutral position and you have difficulty reaching the selected transfer case mode, with the engine running, shift the transmission momentarily to drive and then back to Neutral. This will realign the gear teeth in the transfer case and allow you to complete the shift.
If this doesn't help, you may want to take the vehicle in for an inspection. A certified technician should be able to offer a more accurate diagnosis with a physical inspection.
Keep us posted.
If you have additional questions, please feel free to contact me.
GM Customer Service