Most of Cell chargers should work. Else, you have to create a constant current source. Simple way is to use a 7805 with a 2 diodes in the common path to ground. this will lift the supply to 6.4V approx. on the output put a diode for preventing reverse discharge and add a small resistor to limit the current to say 5mA.
you will find the battery charged in about 5 to 6 hours, as it was not charged earlier.discharge with an LED at say 25 mA until the voltage falls to just 4*0.8=3.2v and repeat the process.