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.