I am planning to make a 4 Degree of Freedom(DOF) robotic arm. I have a confusion in mechanical design of the robot. Shall I use two 4 bar mechanisms to reduce the number of actuators? or a stepper motor on every linkage? Also, a gripper will be attached. So which will have higher payload?
Since a four-bar mechanism is a one dof device, you couldn’t get by with just two four-bars (or else you would end up with a two dof robot).
Four bars can be implemented to reduce the moving mass for a robot. The parallelogram mechanisms that Fanuc made so popular for controlling the elbow rotation are a good example. But you still only get one dof for each four bar.
Regarding payload, etc, there are too many design options to consider to be able to answer. Start with the requirements for your design, including payload, weight, size, cost, accuracy, and allowable controllers, then you can start to address how to implement the design.