Realistically you can start with four if you wanted to. Since more than likely you are going to top up to five gallons in the primary. but you will need to leave enough room for your other ingredients. It is all up to what you want to do. you will have to vary your hops (slightly) with different size boils. But if you are just sarting out it shouldn't make too much of a difference. Good Luck
"Yes, I am a pirate two hundred years too late. The cannons don't thunder there's nothin' to plunder,
I'm an [under] forty victim of fate, Arriving too late, arriving too late."