I had this very same issue about a month after buying my 156 (it's what led to me joining this forum).
I had a warranty, so took it to their recomended local garage. When I explained the issue the managers eye's literally jumped out of their sockets. I explained about the commonly known swarf issue but he said it can't be that and that it would require a removal of the box, sending off the a specialist for diagnosis and then a rebuild with replacement parts and refiting.
Naively I left the car at the garage, but it was bugging the hell out of me, so I rang the warranty company and explained the whole situation. they told me that even if it was down to swarf they would insist on the diagnostics being performed (obviously including the removal and refit) and if it was indeed found to be swarf that I would be responsible for the whole bill as they only pay for broken parts that aren't subject to wear and tear (pointless warranty really!).
I rang the garage imediatley and told them not to touch my car with so much as a screw driver and went to pick it up. I then went to Halfords to buy some axle stands, sealant and gearbox oil to do the job myself following Jamie's rather excellent guide. On my way home I happened across a flock of rabbit's in the road, many of which ended up bouncing round under the car. At that very moment 4th gear returned!! And I'm happy to say it's still there over 8 month on, I've not had to touch it, no new parts, no need to fix anything and I have the bonus of having the bits I need if it does go again.
So I guess you need to find some bunny rabbit's with no road sense to fix your car, garages just want you to bleed cash...rabbits, well, they just want to bleed because they're really Alfisti...