Why use sessions and not DB?

I am arguing(not really) with one of my partners about the use of sessions. He is new to web programming but has been programming other db related languages for years. He wants to use the db to store certain information and I want to use sessions. One of my arguments against using the db is that it is already getting hit several times per user request. If we go his route, it is 4 more db hits per user request. We are expecting the site to become VERY busy and I am afraid that the additional hits would cause problems. Can someone sell me on using the db for this or give me some ammo to sell him on using sessions? We are going to be using php4 on a *nix box with Mysql as the DB server. Any tips is appreciated. thanks CDitty ps...This is for my personal site not postal.