No, if u have 2x2GB sticks u wont see much difference but if u have 1x4GB stick and add another one then u will see a major fps boost, also if u add high frequency ram sticks u will see improments for ex from 4GB 1333Mhz to 4GB 2400Mhz
It depends on a lot of things... The game, the settings, the resolution, what speed of RAM you're putting in, etc etc.
It totally varies from game to game.
Are you using the APUs iGPU? If so go to 8GB should give a healthy boost, as right now a portion of DRAM is dedicated to the iGPU so processing of data for the game or whatever has a limited amount of DRAM. Upgrading to 8GB will provide more for your overall system and also maybe more to the iGPU which would be an additional performance burst
2x4gb 1866 sticks in dual channel mode should give decent performance.
What would really make a difference is an aftermarket cooler & overclocking the GPU portion of the chip if your motherboard is up to the job.
You can push a 30-40% fps increase this way.