The S and the P stand for Series and Parallel. So, a 3S1P 1000mAh pack has 3 cells in series, making a 11.1v 1000mAh battery (assuming its a Lipo pack). A 1S2P 2000mAh pack would have 2 cells, but they would be put into parallel, which does not increase the voltage, but would double the capacity. This would mean that it actually had two 1000mAh cells in the battery. So, a 4S2P pack actually has 8 cells, and in this case, each cell has 2100mAh. It has two sets of 4 cells in series that are then put into parallel. I wish I could make a diagram, because it is so much easier for me to explain it that way. Anyways, I hope that this answers your question. A 4S2P pack will act just like any other 4S pack, its just that it actually has 8 cells inside it.
I bought this battery for my traxxas slash to upgrade from my 7 cell nimh batteries & WOW...WHAT A DIFFERENCE!!!!!!!! (keep in mind that the stock esc cannot handle this many volts) this battery has great power & runtime for a great price! I was afraid of lipos because of the fire hazzard so I went with these life batteries & I'm very pleased! you'll need a lifep04 compatable charger, I use the thunder AC6 & it works great!