ReadyNAS Pro 4 - Stuck in "booting" - Cannot enter boot mode

Does the data shown in RAIDar look right (model number and OS version)?

The drive(s) is (are) not necessarily dead, but at least one likely has a problem that's preventing booting.  Do you have the capability of testing the drives by hooking each up to a PC and using vendor's tools?  If you can pinpoint the bad one (and it is just one, which seems likely due to the circumstances), then you may be able to boot up with just the other three.

There is still a chance the issue is with the chassis, probably the SATA subsystem.  To check if it's that, you can install just a spare drive (not part of your existing volume or containing anything you care about) and see if the NAS can create a volume on it.  If it can, then  power down afterward and move the drive to the next bay and re-boot, repeating for the other bays.  Note that this is just a test to help determine the next steps.  You shouldn't try to put the original drives back in and boot until we figure out those steps so no additional issues are created.

Are you still running OS4.2.x or did you update to OS6.  Some steps beyond this will differ depending on which it is.