I wouldn't put four dive tanks up by the mast. I'd stow them as low and close to the center of the boat as possible. They're heavy. Adding them near the mast base is going to raise the center of gravity of the boat slightly....putting them down below will not.
We have tied ours off the stern (we had a transom). I think you could just secure them to the mast with line. You owuld want to make sure the valves were either covered or out of the way of any swinging gear for about 3200 reasons I think you can guess (assuming you are transferring them full).
SD is right... that is a lot of weight top side. You could also put them in a berth wrapped with blankets, etc.
Since I do not know your destination(s) and intentions for the tanks, whether they will be transferred full, nor the boat, I am not sure if I am giving you good advice - just thoughts. Depending on your destination, you could just leave the tanks and rent them when you arrive?? We have had bad experiences renting regulators and BC's and have always preferred our own. Tanks are a different story.
Are you PADI? Could you just rent them on arrival?
PS No way we could get them in our bilge (WAY TOO BIG) and never liked the thought of them banging around down below or in a lazarette. Most of the other people I have seen find a way to secure them on the transom or have special mounts made for them.