Hi,
USB puts out 5 volts. You need a bit more than that to properly charge four cells in series. So you'd have to do two at a time, or build two chargers to do four cells at the same time. I cant recommend this however, i would recommend using a small wall wart.
Just to note, you can calculate your required source voltage and required series resistance for a tapered charge as follows:
The three equations are:
R=N*(Vbegin-Vend)/(Iend-Ibegin), and
Vs=Vend*N+Iend*R
Pr=2*Ibegin^2*R
where
R is the required series resistance,
Vs is the required DC source voltage,
Vbegin is the beginning cell voltage,
Vend is the ending cell voltage,
Ibegin is the beginning charge current,
Iend is the ending charge current,
Pr is the power of the resistor R.
For example:
Vbegin=1.0 volts
Vend=1.55 volts
Ibegin=0.500 amps
Iend=0.150 amps
N=4 for four cells
Using those parameters we would get:
R=6.2857 ohms
Vs=7.1429 volts
Pr=2*0.5^2*6.3=3.15 watts
So we'd set up the regulator to put out 7.14 volts and use a series resistance of around 6.3 ohms. The required resistor power rating is about 3 watts.