When i am rollin down the road in neutral at 40mph give or take i go to put it in 4th and it just makes this grinding noise and wont let me go in..Any other gear i can go into. When im in third i can still shift to 4th and that works but lately it hasnt let me go into 4th when im in neutral..

 

Something bad inside the tranny. Either broken bits blocking the 4th gear selector gate or a damaged shifter gate.

Dave

 

^^^^^^^ YUP what he said