Ford mark numbers are very subject to urban myth - many will tell you that the Cortina mk5 was the last model but Ford themselves will tell you they only made 4 marks.
Ford Fiesta 2003 - Mark 5 or 6 - MrEckerslikefromRamsbottom
I am under the impression that 'Mk' numbers for the American market Fiestas varied from those for the UK market. A 2003 UK model is generally considered 'Mk5'.
Anything to do with cars built/sold both sides of the "pond" can be confusing - model years are defined differently and production/sale start/end dates can be way different - sometimes two years difference.