Are you trying to measure a single cell (~0.5volt) or a solar panel.
The Arduino should have no problems measuring 0-4mA if you use the right shunt resistor and 1.1volt Aref.
A 100ohm resistor should have 0.4volt across with 4mA cell current.
You should measure current and voltage (two three analogue inputs).
Post a diagram, showing how you have connected things.
Leo..