Just had this problem on my 156 JTS
4th became difficult to select then started to intermitantly jam up.
The link above is spot on, didn't fancy the job myself though so found a local Alfa specialist who took the job on.
For £87 he removed the gearbox end plate without having to take the box out of the car and removed the buildup of swarf and in my case a tiny piece of metal (size of a ballpoint pen ball) which was most likely once a tooth, probably off of reverse though difficult to tell due to being mangled where i had been jamming it against the casing (which also had marks in it!!) trying to get it in gear.
Couldn't believe how small it was to have jammed it up like that but the tolerances for the rod moving to 4th must be tiny!
Don't get conned into having someone take the box out for £500 or whatever they will want, it's just not necessary.
Since doing this the gearbox is a silky smooth shift once more.
You could try just flushing the gearbox first but i did this and it didn't work, presumably the offending piece of metal was too big/heavy to float in the oil or was getting caught on something.