RE: Kadet Senior to Elec.
Typically, you need to combine smaller packs in series. More commonly, you can find 8-cell or 10-cells packs at most battery vendors or you can make your own packs from single cells.
[link=http://www.flydma.com/batteries/batteries.asp?path1=batteries]Diversity Model Aircraft [/link] sells larger GP3300 packs up to 24 cells.
I'm not sure what a 4820 is...