That cell has a MPP point of 3.4V so its hardly any use for charging a 3.6V battery.
You want a cell with a MPP of about 5V (open circuit therefore more like 7V) and limit the output
voltage with an LDO regulator. The diode and LDO regulator together will drop around 1V--1.5V,
which is why 5V at MPP is the sort of spec for the PV array (ie about 10 cells if silicon).
Remember the MPP point for bright sunlight is higher than that for overcast conditions, so if
you want charging to happen on dull days you might want to consider even higher voltages,
or use of a DC-DC converter for generating the 4V output. Ideally a DC-DC converter that
could track the MPP for the cell as well as limit output voltage...