When you have split the pack in a seperate 3s and 4s pack you just charge each pack in the normal way.
When you have soldered the two packs in series, you need a charger that is capable of charging 7s.
For wiring look here:
http://scriptasylum.com/rc_speed/lipo.html
Scroll down to the last dropdown box and select "Hi Amp 3s +2s generic connector". This will show you how
to wire for 3s and 2s in series, for your case you just need to add one wire more on each side.