Is the total lift of a quadcopter the sum of the lift from each propeller? In this diagram each rotor R1,R2,R3,R4 has an individual lift force of 490 N. All combined, does the total force on this aircraft equal to 1960 N?

The aircraft in question is similar to a quadcopter.
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.