You want a PV panel with about 6V open circuit voltage, not 3.8 - otherwise it just won't charge.
The voltage overhead is needed to allow charging in overcast conditions when the voltage will drop
considerable.
full sun: 6V,
likely voltage overcast 4.5V
loss due to schottky reverse blocking diode = -0.4V
max charging voltage LiFePO4 ~ 3.9V
(so even 6V is a little low, as the regulator circuit needs a bit of headroom too)
Any charging circuit has to avoid overvoltage and over current, note. The schottky is required
to prevent the battery discharging heavily through the PV panel in darkness.