You are now running 2 - 4200mah packs in series
Giving you a total of 12 cells (or 4S) at 4200mah total
So to be equal in Lipo you need 2 -2S packs(4200mah) in Series.
As Alan said, with Lipos you never want to run below the 20% of capacity.(damage cells)
I would go with the 4500's or the 5000's
Also you can buy a Low Voltage cutoff, It connects between the ESC and the receiver.
When it sees the voltage down around 12 Volts it will slow you down, but let you drive
it back to the dock.
The Lipos will be faster because they will maintain a higher voltage under load.
Larry