Thanks Mark. I just read a little more of the installation manual. Nothing there about head loss, but the heater is adjustable for pressure, so I'm hoping I'll be OK. Also I noticed there is a paragraph about two-speed pumps:

"TWO-SPEED PUMP:
In a few cases the pressure from a two-speed pump is below the 1- pound minimum required to operate the water pressure switch on the heater. This is apparent when the pressure switch cannot be further adjusted. In these cases the pump must be run at high speed to operate the heater. If the pump and piping arrangement are such that the required 1-pound minimum pressure cannot be obtained, do not attempt to operate the heater. Correct the installation."

I assume by "pound" they mean "PSI" so given that I am at 4 psi on low, I should be OK I guess.

Thank you again to all for your help! Pool Forum is a great resource!