How to mesure lipo batterry while charging

Your results suggest that you are overcharging the cells and not charging correctly.

Generally you should charge at constant current until you reach some threshold voltage and then switch over to (current limited) constant voltage charging.  Many chargers and battery packs do not do this correctly. If you are charging 4 cells in series you must reconcile yourself to undercharging three of them or using active balancing. By the time you are getting close to the end voltage the current should be so low that the difference between volts under charge and not charging should be very small.

MK

The main questin is how i can get accurate cell voltage while charging it. as you can see in my mesurments the first cell is 4.3 volts and others ~4.1 while charging, but when they are not charging, then the first cell is lowest. I think the internal resistances are different in each cell.

when i get accurate reading, then i can start discharging te 2. 3. and 4. cell

You're up against the killer problem with charging cells in series - you need to balance all the time or back off the current much sooner than you would like. You can't measure the battery voltage while charging at high current but you can estimate it. The cells will all be different.

Is this a one off project or for mass production ?

The simple way to manage multiple cells is in parallel but of course the currents can get very high. In an application that I worked on we found dealing with 25-30A currents a much better solution than putting the cells in series.

MK

This is one off project for hobby. I know that I have to balance the cells. But the microcontroller cant do the balancing if it doesn t know the right voltages of cells. I have standart lipo battery where all cells are connected in series. and the balancing lead can not handle the high current that charging needs ( 3 amps in my case ). I am now trying to mesure thebattery internal rsistance before the charging starts I hope that way i can calculate the real voltage of the cell and balance it then.                            

As Michael indicates, the last phase of charging should be constant voltage - in your case 4.2 volts across each battery. Conceptually this can be achieved by connecting a 4.2 volt zener diode across each battery. Zener diodes are probably not quite accurate enough for this and you need to worry about how much power is being dissipated and maybe how much is being wasted.

If you have access to all battery leads, you might be able to hook up 3 separate floating chargers, one on each cell.

As Michael indicates, the last phase of charging should be constant voltage - in your case 4.2 volts across each battery. Conceptually this can be achieved by connecting a 4.2 volt zener diode across each battery. Zener diodes are probably not quite accurate enough for this and you need to worry about how much power is being dissipated and maybe how much is being wasted.

If you have access to all battery leads, you might be able to hook up 3 separate floating chargers, one on each cell.