Really easy to install, Instead of all these fancy valves, I just put my pool on a timer - on at 9:00 when the sun hits the panels, off at 6:00 when it leaves the panels, the water coming out on a sunny day is very warm, almost hot.
2 4x20's should do you fine.
I made mine so I can disconnect two unions and completely remove the panels from the equation, but you can achieve the same with two directional valves and it would be less work.
The reason you may want to remove the panels from the equation (bypass) is that you get better back pressure for your vac without the panels on the system, or on a cloudy, rainy day you may want to circulate the water without running it through the panels since they can also remove heat from the water if the air temp is lower than the water temp and sun is not warming the panels.
If you also want to circulate the water at night is another reason to bypass...