I spotted a book on bird folklore in the library months ago. Though can't remember what it was called. The library might help, I'm a big believer in libraries.
Here are two titles I spotted on abe:
All The Birds Of The Air: the Names, Lore and Literature of British Birds.
Greenoak, Francesca (I think this is quite recent)
Folklore And Provincial Names Of British Birds
Swainson, C. (this is victorian)

If you'd like me to make some up, I'd say it was a positive thing to see four buzzards, because they're a majestic bird that's made a real comeback of late. And if there were four, perhaps that was two parents and two offspring - so that has connotations of fertility does it not.

I try to be rational and scientific but still end up seeing superstitious omenlike things in all sorts of crap (even if I then override what I'm thinking and dismiss it). Maybe that's just human nature.

Just to echo Rhiannon's comment on All the Birds of the Air; this is an excellent book if you can get hold of it (found mine in a skip in Coggeshall :-) The book is full of the names, lore and literature of British birds. Though the entry for the buzzard has little on its folklore it does list its various names and its hunting habits. The entry also says that, "By far the most fitting and descriptive name for the Buzzard is Gled, the glider, from Old English derivation... Watching the slow graceful gliding on the thermals it is hard to believe that the Buzzard does not itself take delight in its actions..."