Loads of people lose them, it's a very common problem
A bloke I work with lost a couple on his XR4 within weeks of owning it, he took his car in to Metro Ford in Fortitude Valley here in Brisbane for a totally unrelated matter and while he was there he just mentioned the wheel caps fell off ....
Metro Ford replaced not only the 2 missing ones but also the other 2 which had not yet gone missing under warranty with brand new ones which apparantly are a much tighter fit than the original ones ... well done Metro! ... nice to hear of good customer service from a Ford Dealer for a change!
check these other discussions here on the XR4 centre cap problem ..
http://www.fordfiestaxr4.com/forums/viewtopic.php?t=768
http://www.fordxr4.com/forum/index.php/topic,561.0.html