If you are heavy or light on the 4", you should adjust accordingly, and always, always buy extra, you don't want to run short for something that important. I'd recommend and overage of 1/2 up to 1 yard. Come up with a plan for some of the extra if you can. I've been to a few jobs where the slab had to get made smaller suddenly, and it always is a major drag. The extra hundred bucks is well worth it for your driveway! Disregard the mesh in calculations. Make sure and put plenty of relief joints in the slab, and consider using fiber reinforcement, and a 6 bag mix is worth it, too afa I'm concerned.