I am a drip irrigation hobbyist rather than an "expert," but I'll tell you what my experience has been.

1) For one, I would certainly put the backflow-preventer, filter, and pressure regulator downstream of the timer, as there will be additional pressure losses through the valves and turns of the timer. So yes, that means a backflow-preventer, filter, and pressure regulator for each line, in order.

I actually had high-quality hose bibs with built-in backflow prevention installed about a decade ago during some remodeling work, so now I only need a backflow device if working on a system for other people. Many municipalities are requiring backflow prevention hose bibs to protect the water supply from contaminants, so this might be something to consider as well.

2) So yes, by using the pressure regulator after the timer, lines 3 and 4 can be attached to regular garden hoses at full pressure.

3) I routinely use high-quality garden hose (contractor-grade hoses, or Craftsmen black all-rubber hoses) to transport water to my drip irrigation systems that are 100' or more from my water sources. Since you are limited on the run-length of the 1/2" poly tubing due to static pressure loss along the length of the run, using a garden hose to get to the area to be irrigated is the only way I know how, short of installing a new water line.

-Kris

Thanks - very helpful info!

I should have checked the product descriptions for the backflow-preventer, filter, and pressure regulator from the online store: they shouldn't be subjected to the constant static pressure they'd be under if they were installed before the timer.