readynas ultra 4 and wd disks?

The Duo and Ultra are very different devices with very different hardware including CPUs of different architectures. Do not assume that hardware that works with one will work with another. Due to the increasing variety in ReadyNAS models NetGear chose to split the Hard Disk HCL to have a separate list for each ReadyNAS model. If they didn't do this the ability to qualify drives would have been severely affected. There can be a big difference in what is compatible most noticeably between the Duo (2-bay) and 4200 (12-bay rackmount).

There is a community hardware compatibility list but I would not trust it for something as important as the internal hard drives in the ReadyNAS. NetGear extensively tests disks and whilst it's possible a drive may not be on the list because it hasn't been tested yet or testing is in progress, it is possible that there may be compatibility issues.