Amount of kickback from dc motor
The 10 uf 50V capacitor you are talking about looks like its connected to the Power rails, and it is used to smooth out the power lines. Depending on your power supply and voltage you are using you can switch out the cap for another value, I would suggest at least double the voltage and a similar capacity. Check out this question for a better explanation.
 Decoupling capacitors: what size and how many?

the kickback that you are talking about is usually quite high and is taken care of by the motor driver, not something you need to worry about in this example,but double check the datasheet to be sure. when creating your own motor driver you will need to consider these conditions.

as for your 4V motor, best way to measure is hook it up to a 4V source with an multimeter in series and turn it to measure milli amps and run it for a few seconds and get the running current, next if possible stall the motor for no more then 2 secs and read the current. That will tell you your running current and stall current.