I got pretty good results today. The 4 Watt panel delivered 4 Watts at full sunlight and the battery is being charged at 3.5 Watts, about 88% efficiency.

That is almost a Watt more than the previous solution where I simply placed a Drok buck converter from Amazon between the solar panel and the battery.

MPPT Solar Charger from Buck Converter-Fail

I turned the potentiometer by hand until I found the maximum power. Here is the whole setup:

Here is the power curve (a bit later at about 1340 W/m^2)

A few more tests before I turn over control of the PWM to the Arduino!