Worn syncros on gear and slider -> the slider does not engage with syncroring and gear, and feels like hitting a brick wall.

Remember the syncro mechanism consist of three parts; gear, syncroring and slider.

If its crunching its the syncroring that is not able to accellerate / decellerate the gear due to friction surface wear, and the slider and gear collide at different speeds -> worn syncroring.

I think you need to replace the input shaft (4th gear) and syncro sliding hub 3-4 (slider).