I would just replace the 2 on the same axle and then make sure the better set is on the rears.
Could you take a photo of the sidewall, try scrubbing it down first just to see what its really like. Have you asked a garage to give their opinion on if they need doing.
You could take the opportunity to go upto 15" wheels. I even have 4 15" spare that would fit that car, at this moment though they are not particularly nice looking at all (stock)