Is Khums paid on difference of this years savings and last years savings (if last year was already paid)? For example, if after my expenses this year I have $5000 left, will I pay 20% Khums on this total $5000? Or if I had $4000 left last year, will I pay Khums on the $1000 this year ($5000-$4000).

You should pay the khoms of 1000 only