FYI
I had my car brought back to VW yesterday with the same issue: misfiring in the fourth cylinder.
I have been in contact with Fair Trading, who advised to wait and see what is done during this service. If it is the same fix that was done the previous 3 times (spark plug and coil back replacement), I will complain to the dealership for a replacement/refund as they do not know how to fix the issue. If that fails, I have been advised to raise a complaint through the Fair Trading website so they can get in contact with the dealer.
I will be drafting a letter to VAG to voice my concerns tonight.
This is just so sad because the car is so good when it works..... Driving a loan Golf TSI wagon at the moment, lets hope I lasts long enough to get my Polo back.