YOu don't have to use 4Fg powder for prime. You can do just as well with 3Fg. So, first decide what powder you want to use in your rifle or smoothbore, and then determine the need for priming powder. But, as Roundball has indicated it really becomes a matter of math. You have to have some idea of how much powder you use when you prime. I use more than 3 grains.
If I were ordering powder for the long haul, I would order a case, with 23 lbs. of 2Fg, and 2 lbs. of 3Fg. I happen to use the 2Fg powder in both my smoothbore, and my .50 cal. rifle, but the 3Fg can be used in the rifle, too.
I do think that Roundball's suggestion to order a full case, to gain further savings, is smart. Also, check with Graf and Sons, and with Powder, Inc, two other distributors mentioned here often my members as great places to order powder at rock-bottom prices. All three mix an order however you want, and it is delivered to your doorstep. You can find a list of powder distributors by going to the Member resources section shown on the Index page in the top, or first section.
Do some comparison shopping, buy what you want. People are also raving about the quality of Swiss powders, and of both Schuetzen powder, and Wano powder, which they are buying from one of the distributors cheaper than the price of Goex. Swiss is more expensive.
I think you might also consider buying and storing a second case. If the poop really does hit the air conditioner, you will own some very expensive product that will be in high demand. The richest man is the man with friends, and a man who takes care of his friends need for powder probably will have the best friends of all by his side. He will need them.
Ast to storage. If you do not open the cans as they are delivered from the factory, you should have no problem with moisture contaminating the powder. Now, being make of metal, moisture can rust the outsides. So, putting the cans in a couple of layers of plastic bags would protect the cans from exposure to moisture. Use some dessicant packets in each bag to help absorb any moisture that might get in. Just store the powder in a COOL, DRY place- not in a damp basement, or in the garage where it hot as blazes in the summer, and the humidty is high.
If you own an old refrigerator, it can make great storage place for powder, The sides are insulated, to keep the temperature inside stable, and if the motor and condenser work at all, they will remove the moisture that might get into the box. Removing Ice, or water from the drip tray is about all that you need to considerin periodic maintainance. Just don't store your beer, and other consumables that have water in them in the box. That would be expecting a bit much. You do want to modify the refrigerator door by putting a hasp on it, so you can padlock the refrigerator to keep nosy people and kids out of it.