Several points:
1. Robert Allison: Both oxygen and nitrogen from the air readily dissolve in ground water. As pointed out in a previous post, it's that dissolved oxygen in water that fish breathe to stay alive, even when the lake or river is covered in ice. Since MOST cities get their water from rivers or lakes, the tap water in most cities will contain dissolved oxygen. Some cities like Phoenix, Arizona and Las Vegas, Nevada get their water from an underground aquifer, and I don't know if that water contains any dissolved oxygen. Most tap water also contains some hardness ions (less so in "soft" water and moreso in "hard" water). It's the dissolved oxygen and hardness ions in tap water that make it a "not such a good idea" to change the water in your hot water heating system. Since it's that oxygen that causes rust on the inside of the iron boiler and iron piping, and the hardness ions that form scale on the hottest parts of the heating system (the boiler), you're better off to save the old oxygen depleted and ionically dead water you drain out of your heating system to do a repair, and siphon it back into the heating system after the repair is done, than to simply refill with new oxygen and ion rich water. No plumber would ever do that because you're paying him $90 per hour, and you're not going to want to see him carrying pails of water upstairs for that wage, or standing around not doing much of anything while the siphon is flowing to put that old water back in.
2. So far as the water heater goes, I would simply drain it. They say there is a "glass lined" steel tank inside the water heater, but this "glass lining" is just a porcelain enamel coating very similar to the enamel on a steel bathtub. So, if the water heater is only 4 years old, there shouldn't be any cracks in that enamel coating, and so the steel tank shouldn't rust any faster than a steel bathtub left empty. If you want to dry the tank out, simply unscrew both the drain valve and the pressure relief valve and use a vaccuum cleaner to suck the moist air out of the tank or blow fresh air in periodically.
Remember, cracks have to form in that porcelain enamel coating on the inside of the tank for the steel tank wall to come into contact with water and to start to rust along those crack lines. Otherwise, the water doesn't come into contact with the steel, and you won't have any rust. It's the repeated heating up and cooling down of that enamel coating that causes it to crack.
3. It's common to have two (or more) water heaters plumbed in parallel, but it's not common to see them plumbed in series. The reason for this is that in series, it's the first heater that does the lion's share of the work, and the second heater does very little work. The result is that you're going to be replacing that first heater as often as if it were working by itself. A. O. Smith says as much in their Technical Bulletin on series and parallel plumbing he
http://www.ho****er.com/lit/bulletin/bulletin64.pdf
(this web page edited the URL because of the character string "t-w-a-t" in A. O. Smith's web site.) Replace those stars with the correct letters, and you should find the right technical bulletin.
With two or more water heaters in parallel, the water inlet and outlet manifolds have to be designed so that the resistance to water flow through each heater is the same. This is typically done by making water inlet and outlet manifolds that provide the same resistance to flow for each water heater, like this:
