Transmission issue. Diagnosis needed please.

[EricsCar]

So these are the symptoms:

1. When changing from 3rd to 4th gear, you hear and feel a slight but obvious crunch.
2. If you change slowly, by putting it in neutral for a sec, it goes into 4th with no problem.
3. Same issue going from 5th to 4th, or 6th to 4th.
4. No problems whatsoever going into any other gear.