I thought that's what you were saying, but it didn't quite make sense as written. Most of the chargers I have ask you to confirm the higher cell count.

I'm still confused on your math tho, 18V/5=3.6V, 18/4=4.5. 4S=18v?
If I am understanding you, you say charge as a 4S to 3.6V/cell, then stop, then charge as a 5S. Full charge on 5S gets you to 21V.

If you are charging a 4S pack to 3.6, then stopping, that only gets you to 14.4V. It will still "think" is a 4S pack. That's still less than 3v/cell for a 5S anyway, which is bad to begin with. If you do a 4S full charge, you can get upto 16.8, but as said, I would stop mine @ ~16.5V as it goes into CV mode and the amps slow down alot.
(This was done on a charger that would only trickle charge until voltage got over like 3.3V/cell, annoying as hell. I'd cheat it up to 16.xV faster, then go to 5S mode. I wouldn't do this unless forced to tho, that charger was dumb like that.)

You are quite correct, the charger does ask you to confirm you are (in this example) charging a 5s lipo, but it only 'sees' a 4s lipo due to the low-voltage issue. In which case, my charger will charge the pack as a 4s lipo once you press the OK button (it will ignore you telling it you have a 5s lipo hooked up, even with the balance plug connected). That way, all the cells are charged (since they are in series and the balance plug is only for balancing during charging) and you just have to abort the charge once all five cells reach ~3.6v to give a total of 18v. Then retart it with the correct setting of 5s and they will all charge in unison until they hit 4.2v.

I know it kinda sounded like I was saying 'charge 4 cells up until they reach 3.6v/ 4.2v, then stop it & start it again until they all say 4.2v including cell 5', but what Im really saying is 'charge all 5 cells until they get to 3.6v whilst ignoring what the charger tells you about pack size/cell count, then stop the charge and restart it as a 5s pack until its fully charged'. **breath**

okey dokey ;)