As a Master plumber, I commend you on your effort. It is expensive to install a water heater. I mostly do industrial/heavy commercial, so I don't know all the details of residential. I did work for myself briefly, though, doing this.
When I did it for myself, there was enough time involved with loading/unloading water heaters (to be cost effective, you usually did 3-4/ day, with a helper) that you are running from one to the next, then dumping the old ones all when finished (obviously you don't want the old one, right?). 4 heaters needs to equal 16 hours labor (2 guys, 8 apiece), gas for the vehicle (sorry, a full size van can barely carry 4 OLD water heaters, with tools, parts, etc you'll need to get a new one in anywhere), and considering PREVAILING wage in my area is about 20/hr for a plumber (I make more than that, BTW; I wouldn't do my job for that little. Sorry-I value my time/exp), plus 5/hr for benefits, plus 7/hr for a helper (2/hr benefits?), you must pull a permit for each (here-25/ea. You may not have to, since you did it yourself, but they may require an inspection), a few parts (20/ea), gas (est. 150 miles-My metro is urban spawl paradise-@ 11mi/gal, 3.50/gal cost=50 bucks about), if you charge 200 apiece (also low) you'll make about 300 in profit a day (roughly). Now, I didn't add in any other overhead, like keeping the lights on in your office, the phone on to schedule, internet for invoicing/contacting clients, advertising (yellowpage ad can be 50 dollars/mo starting, ONE billboard 3-600/mo), insuring (or paying for) the van, bonding, renewing your license, etc. But you can see things can go from decent to horrible very quickly, since you're going to have days you aren't fully booked, and you're still going to pay your help (probably yourself, too?) or they won't be sticking around. I'm not wanting to necessarily justify the cost to you, just explaining what figures into it.
These costs are typically higher when you work for yourself, because you can't spread it over a bunch of work being done my many people under you; it's just you. And, I'm sure there are plumbers out there who don't have the stuff they're supposed to have to do their work; i.e., a masters' license (needed here but not everywhere), the permit for the job, bonding to cover the job, etc., to save cost to themselves, maybe a little to you. But, we also want to keep in mind we wouldn't work for free ourselves. You just think if you could do what you just did in under 90 minutes (leaving 30 minutes travel time), 4 times a day, in basements, in attics, in crawlspaces, and then ask yourself how much you would want for your trouble. 40 bucks seem like enough for what you did?
This is why I don't do it anymore; I personally didn't make any money on any of my jobs, and I didn't have much work anyway. But, residental is much different from industrial. I've never pulled PEX in a refinery for running chemicals around. I wouldn't work in one that did. But, everyone wants to pay their bills and earn a fair amount of money. Just don't assume ALL of that installation cost is on the check. Even in big residential plumbing companies, their employees (the plumbers) don't get most of that money; it's mostly overhead. If you can't afford (or don't want) to pay somebody, it doesn't hurt to do it yourself.
