Hope this helps...
A Lithium polymer cell operates between 4.2V when fully charged at 3.0V when fully discharged. It has an average voltage of between 3.6V and 3.8V. You charge them under a regime called CVCC which is constant voltage/constant current. Your charger will be set to charge the cells at a maximum of 4.20V per cell (or 8.4V for a 2-series battery. The remaining advice here assumes a 2-series battery).
It is advisable to limit the current to 1C (in your case 4A). Assuming your battery is discharged when you start the charger the charger will limit the current to 4A and you will notice that the charging voltage will steadily increase towards 8.4V. While the charge voltage is <8.4V you will see that the current is max'd out to the 4A value and this is the time when you are putting the most charge into your battery. Once 8.4V is reached the charger will hold that value and the current will start to taper down towards zero. Many people think that once 8.4V is reaches the cell is fully charged or 'peaked' because they are comparing the effect to NiCd or NiMH cells. This is not the case and the current should be allowed to taper down to at least 0.1C before the charge terminates. I don't know what current taper your charger uses and a charger may terminate any where between 0.1C and 0.025C. As the current is tapering you are putting less charge into the cell for any given unit of time.
Overall with LiPo cells:
*Never charge them over 4.2V/cell (8.4V for a 2-series battery)
*Charge using a LiPo charger that has a balance function.
*Use an ESC with a LiPo cut-off (or always stop your car before it runs slow)
*Never leave them in your car connected - even if the car is switched OFF.
*Charge them in a LiPo sack
*Store them 40-50% charged (my advice is to charge them before your race and then store them with the capacity that remains following your race).
*Stop using them if they swell.
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