I had a very similar problem. Suddenly I could not get 4th or reverse. I took car into garage but it miraculously cured itself when they came to move the car. A few days later the problem came back. Googling the problem I found that someone else had stripped out all the centre console stuff and found a loose bit of metal in the little box area where lever goes into gearbox. This was preventing the gears being selected. I stripped out my centre console and lo and behold there was the loose bit of metal which after removal cures the problem. I think this piece of metal is in fact a bent up projection from the securing plate that helps provide the additional spring pressure required to get into 5th and reverse. I had noticed a while ago that this had gone. What happens is that wear occurs and the bent up projection frees itself from the plate and then rattles around in the box causing gear engagement problems. You would be better of getting a new plate and thereby restoring the spring pressure but if not you can engage all the gears, the box is just a bit vaguer than it was.
