Yes, you can install a 4iiii or Stages R7000 or R8000 power meter on a Shimano GRX crankset as they both use the same interface – which is called Shimano Hollowtech II.
However, there are a few things to note:
1) Most importantly, the GRX crank arm uses a different (wider) offset than the road crank arms (R7000 and R8000). This means your stance will be slightly asymmetrical.
2) The color and the look of the crank arms won’t be exactly the same (a cosmetic issue only), but they are very close so most customers are ok with this.
3) Whenever you are installing a left side power meter crank arm, you need to confirm you have 10mm of clearance between the left crank arm and the chainstay in order for the sensor to clear. Please see the image below.