I'm curios abouth amp!
 

HI
Give the same lipo same amperage at different capacities?
For example. After a while of using my lipo (5000mah 20C) it's capacity drop to 4000mah. Can that lipo still give 100A or it's drop to 80A too?

It is 80, the "C" rating is what multiple of the packs capacity it is safe to discharge a pack at so a 4000mAh = 4Ah 20C pack is good for 4x20=80Amps
if you need a 100Amp capable pack but only want the weight/size of a 4000mAh you need to move up to a 25C pack as 4000mAh = 4Ah, 4Ahx25C=100A

I know how to calculate current but I want to know if a 5000mah 20c lipo keep giving 100amp current when it's capacity drop to 4000mah.

most likely not, apparently the cells are beginning to weakin, so i would say they might not even put out a full 80 amps as of now