A few other things to consider:
The Fiesta is quite a small car if you have a family. Say you commit to one of these newer cars and then in a couple of years another child comes along. You'll then be stuck with a car that you may not be able to afford and a car that is way too small.
Honestly if I were you I'd be looking at something like a mk2 focus (bigger boot than the mk3) or even a mondeo - loads of space for your family even if more kids come along.
Plus as others have said, the fiesta will be worth nothing in 4 years so are you really going to be willing to put 4k in to a car you've already had for four years (I know I wouldnt), providing you can save 4k in the first place.
My advice, get a loan for lots more than you need to get the cheapest interest rate. Then buy a car at a sensible amount of money. Then with what's left, pay it off the loan. The term will then be reduced and you'll still have the low monthly repayments.
This is exactly what we did with my wife's car. We wanted 4k over 3 years - we borrowed 7.5k over 5 years. The monthly payments were less than borrowing 4k over 3years due to the lower interest rate. We then bought a car for 3.5k, spent 500 On putting a new clutch in it and then payed the remaining 3.5k off the loan. This then reduced the term to just over 3 years while still having the nice low monthly repayments. So in a nut shell we got 4k over 3 years for quite a lot less than we should've. It was actually the bank manager that advised us to do it that way. When I thought about it, it was a no brainer.