Dk45s shifting issues

[mountainman4761]

I have a 2009 dk45s with 470 hours on it. I have had some issues with the tractor popping out of 4th gear in the past. It hasn't been a big deal as I rarely use it unless I'm in medium range. Today while using the tractor it would not go into 4th at all and third has become very hard to shift into. First and second gear are fine, the range selector is fine and the forward reverse are fine. I checked the hydraulic oil level, and linkage obstructions and lubricated the linkage. Any thoughts on what it may be?