Hi, it shouldn't actually stop the 0% interest rate, but it may mean that the purchase amount on the card starts to accrue interest, and depending on the interest rate your purchases are on,it may mean you get into a sticky situation. Just don't worry. All I would advise is to call up and check because when I worked for them up until a few months ago, there were instances whereby you do a 0% Balance transfer, make a purchase at 15.9%apr, then you start accruing interest on the purchase and then the payments will get allocated to any promo rates first, so that'd mean your 0% bt payment would have to be cleared before you even touched upon that £4. Although, if it was a genuine mistake, sometimes that can be resolved and a small amount such as £4 MAY be able to be included into the 0% promo rate as courtesy so you avoid interest.Although that depends what they say when you call up as it wasn't something that is done all the time, only in certain circumstances.