RAMPS 1.4 losing power with more than 3 drivers mounted
Your power supply may not be able to provide enough current. Check it's output voltage, the input at the RAMPS and the supply voltage (both VMOT and VDD) on each driver.

If the supply is good, you can suspect the polyfuses on the RAMPS board. These are yellow rectangles, designed to protect against over-current. They age with stress, and typically fail to pass their full rated current (but you need to understand if there is a fault which damaged them). The link describes how to remove them.