Yes, you can install a Shimano R8000 power meter on a Shimano 6800 crankset such as 4iiii PRECISION Shimano Ultegra R8000 or the Stages Shimano Ultegra R8000 as they both use the same interface – which is called Shimano Hollowtech II.
Please note, the color 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.
Please note, these power meters attach a sensor to the inside of the left crank arm. On some bikes, there is not enough room between the left crank arm and the chainstay for the sensor to clear the frame. Therefore, you should make sure you have at least 10 mm of clearance for Stages and 4iiii PRECISION 2 power meters. Also, please make sure that you don’t have a bottom bracket mounted rear brake (used on some aero and TT bikes). Please refer to the image below.