As Christy said, the F3 is the basic one, the F4 is the all inclusive one, which most Cattery's/Pet Resorts will reqiure you to have in case you ever want to give Kitty a Holiday! There's usually only about $20 differnece between the F3 & F4 - 1 shot for the initial vaccination, then a booster a few weeks later, then just a booster once a year... The cost varies form vet to vet but are usually well under $100 per shot depending on where you go... My Kitties are both done, they are mostly indoor cats right now but you never know what you or your guests can track in on your shoes etc, I just get mine done as I'd prefer the yearly shots to the very small risk of maybe watching them die painfully as I didn't get the shot Prob not much of a risk to the indoor cats, but I am very much a 'just in case' kinda girl!

Good luck!

Alicia xox