Sounds like you have syncro issues which means you'll have to get it looked at. Good gearbox oil can help depending on how bad it is, so perhaps try that first. I'd recommend Redline but depending on the workshop they may offer another option.

Generally speaking, if your syncros start crunching your syncro rings are loose and the gear isn't engaging correctly. This will require it to be opened up and fixed = big job and big $.

I just had my gearbox opened up recently from a crunching 4th gear in my GTI and it was a long and expensive exercise.

Red Line Synthetic Oil - Gear Oil for Manual Transmissions