eh - not sure it will matter much. 5 gallons vs 4 gallons still has the same sugar content. If you pitch so many cells per degree plato per gallon, its pretty much identical at average gravities. eg Mr Malty has it 180 vs 182
Describe the sour taste a little more. Like vinegar sour or like green apples?
Despite what your kit instructions may have said you didn't need to transfer to secondary after a week. In fact, you don't need to transfer to a secondary at all and your beer would probably have been better off staying in the primary for 3-4 weeks and then bottling.
You may just have what's called "green" beer on your hands. It very well might improve as it bottle conditions. How long has it been in bottles? You should really give them 21 days in the bottle at 70F before you start judging them. If they're colder than 70F you should give them a little longer than that.
From "How To Brew":
A flavor of green apples or freshly cut pumpkin; it is an intermediate compound in the formation of alcohol. Some yeast strains produce more than others, but generally it's presence indicates that the beer is too young and needs more time to condition.