OK,
I have a very simple answer to whether you should run your panels or not:
If the water coming from the panels is warmer than the pool, by even the slightest amount, you should run them. BTUs are what you want, temp is less important. ANY warming of your pool is A Good Thing, and every BTU you can grab from the panels brings you closer to your target warmth.

A 4lb increase is not much at all--be happy!