Decreasing lumen and power usage
This can occur for several reasons.  My first observation would be whether the application/installation is the same for all four lamps.  If one is in a recessed can, covered, or enclosed, it may produce less light.  LEDs experience a "thermal roll off": as the LEDs heat up, they become less and less efficient.  

Typical light bulbs experience a 6-15% roll off as they warm up and stabilize.  If one bulb rolls off more than the rest, it can be due to being placed in a hotter environment OR perhaps they assembly was not consistent and did not secure the LEDs to the heat sink/thermal solution.  

Lastly, there may be an issue with the driver (power supply) inside the light bulb.  If certain components fail open or disconnect, there can be a drop in power consumption.  If they are producing far less light as 40% suggests, they most likely are consuming much less power.  What causes that drop in power could be a number of things.