yes, you can do it. But not perphaps one at a time, you have to replece them each single drive step by step. Otherweise your RAID will be lost if you are in RAID 5 mode.
replace the first drive, run the RAID rebuild
replace the second drive, runt the RAID rebuild
replace the third drive, runt the RAID rebuild
replace the fourth drive, runt the RAID rebuild
Now you can enhance the RAID to the new capacity.
Best practice is, to connect the My Cloud to an Uninterruptable Power Supply. If power cut would happen during the so called RAID grow at the end, it could end up in a catastrophy. UPS does not need to be huge, a 650 or 700 VA is more than enough if you connect only the My Cloud device. Even a battery-power-socket would help to prevent short power cuts.