Pete,
Bring me another bag of coffee and I'll tell you exactly what you want to hear. You can even try my backhoe to set the final hook so you buy one!
But back on a serious note.....
As for trenchers, I rented a walk behind for running an electric line and and would not recommend assuming your soil is as rock-laden as ours. The grapefruit sized rocks are fine, but when they approach the size of a dog the rock will win. Also, I couldn't get any deeper than 18-24 inches. 4 feet would have been out of the question. Maybe a bigger unit would work better, but the really big rocks will still be a problem.
I dug my water line with a 16" bucket and kind of liked the size since I could walk down the trench to roll out the pipe. I don't think the larger width slows the digging, but it does make a bigger scar.
It's not hard to learn either. A bit clumsey at first, and the advise to run slow while you're learning is well worth taking.