I haven't seen any pictures on a 4x20... even when sold as a 4x20 it mentions 2x2x20.

BTW.. the panels I got from ebay were missing the internal diverter that forced water to go in a U shape in a 2x20 section. I suggest you test them on the ground first.

In the sun, the panels will get hot. When water is running through them, they will not be hot. It's easy to tell they're working (no hot spots)!

The physics says you'll get the most BTU transfer with the least delta-T(emp) (moving more water through it) .. that's why parallel is better than serial flow (you'll also have less head pressure)

Per instructions (at least for my panels)
Code:
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   ||>>>>>>>>>>>>>>>>||
   ||<<<<<<<<<<<<<<<<====pump

Theoretical Best
Code:
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   ||                ||
   ||>>>>>>>>>>>>>>>>||
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   ||                ||
   ====pump          ===return
I cabled it up like this (using parts that came with the kit.. I didn't have a second couple for the theoretical best
Code:
   ||>>>>>>>>>>>>>>>>====return
   ||>>>>>>>>>>>>>>>>||
   ||                                      
   ||<<<<<<<<<<<<<<<<||
   ||<<<<<<<<<<<<<<<<====pump
I don't recommend you knock yourself out trying to get the "best". The differences are probably negligable. Go for what's easiest to do. I adjust the flow through the panels to limit the temp rise to 10-15 degrees. Enough to make me feel the panels are working even thow the efficiency is lower than if I ran more water through it.

Robert