Is the total lift of a quadcopter the sum of the lift from each propeller?
Yes.  490 + 490 + 490 + 490 = 1960.  For practical purposes, the thrusts from individual rotors do not interfere constructively or destructively:  they neither cancel each other out, nor do they help each other.  This case is pretty much the same as multiple propellers mounted on a fixed wing.

The situation becomes more complicated only if some rotors overlap, or are coaxial.