Try to increase Rays per pixel and Rays bundle size parameters.
Default values of those parameters are optimized for single/dual card machines, for more devices adjustment might be necessary.
Please check with 3rd part tool like MSI Afterburner or EVGA Precision.
Ive seem the task manager reporting wrong GPU usage on many occasions.
If they report low usage as well, can you please send the scene with which this happens to support@chaosgroup.com?
Thank you Blago, I have tried with asus GPU tweak II, and it is showing all 4 cards with GPU usage 100% - thank you for your support, I won’t have any trust on task manager from now on… Have a great day!