[A4] To change or Not to change all four tires

[Venkat Venkataratnam]

Hello,

I have '98 A4 2.8 Quattro. I just had a flat on one of my tires (rear one) 
on the side wall. Should I change just the two rear ones or all the four? 
The rear two might have lasted just couple of more months.

The front two are in very good shape. I would hate to replace them.

Venkat