OK, confused - pic shows a series setup, yet explanation sounds somewhat like a parallel setup.
Need to know which way it is.
What you describe with the heatup of the panels (and it sounds like you also have a mix of different panels) makes it seem like a parallel setup where water will take the path of least resistance (the cool panels) and the other panels stay stagnant and heat up.

The way you have to set it up is as follows:
water from skimmer to pump, from pump to filter, from filter to INPUT on SP1, from OUTPUT on SP1 to INPUT on SP2, From OUTPUT on SP2 to INPUT on SP3, from OUTPUT on SP3 to INPUT on SP4, from OUTPUT on SP4 to return jet.
This is the ONLY way you will get water moving ewenly through all panels.
If you run in parallel, there is no guarantee you will get the water moving through all panels.