I believe this is normal, I have the 2 bay unit and that one took a few hours to initalize. I’m guessing that since the drives are new, they must be setup in the RAID. This should only happen once. Unless you loose power, then it could take up to 5 days for the 4 drive units according to other posts. This is why you should use a UPS and also get the 2nd power brick.
If this unit had a high end RAID controllers then the initization or rebuild would be a lot faster, but that would cost a lot more and for this form factor, it would not fit.
