Make sure you're in neutral. Sometimes things have to synchronize. Roll it backwards/forwards a little and try again. Try with the engine off. 203s can be a real bitch if heavily worn as well. Also make sure you don't have different gear ratios front/rear. Uneven tire wear or mismatched tires can make a slight enough difference especially on dry pavement or hard pack to get it out of 4-lo once you get it in. We had an old work truck the rear tires hadn't been rotated in a long time and once it got in 4-lo we like to never got it out. It was a matter of getting it in looser dirt and rolling forward/back/kick the trans in neutral and quickly try to pull it out of 4-lo.
