Brett,
If original, they are the old 'tapered plug' type, solid bronze, 35+ years old and would probably cost you a fortune to replace, since they are now almost antiques! Long ago I tried to get a couple of mine to seal using valve grinding compound and finally gave up.
My advice is to replace those 4 plus the rest with modern Apollo, Groco or similar 1/4 turn seacocks. It is not a lot of money, if you do it yourself, and will provide you with priceless peace of mind.
Dave