Data recovery posibility disk 1 failed hard drive, disk 4 sectors that could not be read

I'm posting on this forum to get further details to consider if Netgear support is likely to recover data after the following events occurred written below. The device is a ReadyNAS 314, has four hard drives. The volume created using Flex-RAID RAID 5. The ReadyNASOS is version=6.4.2, arch=x86_64.

Data stored on the NAS was archived again on another share on the same NAS device using the web admin backup interface. On copying, the files to the new share on the NAS the errors below was recorded. Disk one was stated to be Degraded then stated to be Redundant.

notice:backup:LOGMSG_SUCCESS_BACKUP Successfully completed backup job Backup Storage.
warning:volume:LOGMSG_HEALTH_VOLUME Volume data health changed from Redundant to Degraded.
warning:disk:LOGMSG_DELETE_DISK Disk Model:WDC WD40EFRX-68WT0N0 Serial:WD-WCC4E6FY4T46 was removed from Channel 1 of the head unit.
notice:disk:LOGMSG_ADD_DISK Disk Model:unknown Serial:unknown was added to Channel 1 of the head unit.
warning:disk:LOGMSG_DELETE_DISK Disk Model:unknown Serial:unknown was removed from Channel 1 of the head unit.
notice:disk:LOGMSG_ADD_DISK Disk Model:WDC WD40EFRX-68WT0N0 Serial:WD-WCC4E6FY4T46 was added to Channel 1 of the head unit.
notice:volume:LOGMSG_RESILVERSTARTED_VOLUME Resyncing started for Volume data.
notice:disk:LOGMSG_ZFS_DISK_STATUS_CHANGED Disk in channel 1 (Internal) changed state from REMOVED to RESYNC.
notice:volume:LOGMSG_RESILVERCOMPLETE_VOLUME Volume data is resynced.
notice:volume:LOGMSG_HEALTH_VOLUME Volume data health changed from Degraded to Redundant.

The next day the following error was recorded

warning:volume:LOGMSG_HEALTH_VOLUME Volume data health changed from Redundant to Degraded.
err:disk:LOGMSG_ZFS_DISK_STATUS_CHANGED Disk in channel 1 (Internal) changed state from ONLINE to FAILED.
warning:volume:LOGMSG_HEALTH_VOLUME_WARN Volume data is Degraded.

The disk 1 wasn't replaced, the NAS then became unresponsive when the following errors were recorded.

err:disk:LOGMSG_ZFS_DISK_STATUS_CHANGED Disk in channel 4 (Internal) changed state from ONLINE to FAILED.
warning:volume:LOGMSG_HEALTH_VOLUME_WARN Volume data is Degraded.

Disk 1 that was reported to have failed was taken out and reinserted again.

warning:disk:LOGMSG_DELETE_DISK Disk Model:WDC WD40EFRX-68WT0N0 Serial:WD-WCC4E6FY4T46 was removed from Channel 1 of the head unit.
notice:disk:LOGMSG_ADD_DISK Disk Model:WDC WD40EFRX-68WT0N0 Serial:WD-WCC4E6FY4T46 was added to Channel 1 of the head unit.
notice:volume:LOGMSG_RESILVERSTARTED_VOLUME Resyncing started for Volume data.
notice:volume:LOGMSG_RESILVERCOMPLETE_DEGRADED_VOLUME The resync operation finished on volume data. However, the volume is still degraded.

The disks were only in operation for a year and 6 months. The NAS being unresponsive after errors recorded on disk 4 an attempt to power it down the power button was press twice to shut it down. It did not after 20 mins and a hard shut down was done. When starting up the ReadyNAS it was responsive but the volume wasn't there anymore. The 4 hard disks all had a green indicator looking at the web admin interface instead of the yellow indicators for disk 1 and 4.

Speaking to a support staff at Netgear the first person spoken too had the logs sent over to them. After looking over them the support staff stated the data could be recovered by no guarantees. It was explained they would remotely log in and copy some data from maybe off disk 4 as it's partly failed. Then replace the either disk one as it completely failed and regenerate the data. Or replace both disks and regenerate the data. Couldn't get more details as only after making the payment would it be sent to another support take who can take a better look. Viewing at the disk_info log can it be seen if data recovery is possible? Would like to get some feedback before making payment.

Device: sda
Controller: 0
Channel: 0
Model: WDC WD40EFRX-68WT0N0
Serial: WD-WCC4E6FY4T46
Firmware: 82.00A82
Class: SATA
RPM: 5400
Sectors: 7814037168
Pool: data-0
PoolType: RAID 5
PoolState: 5
PoolHostId: 7c6ee44a
Health data
ATA Error Count: 15
Reallocated Sectors: 0
Reallocation Events: 0
Spin Retry Count: 0
Current Pending Sector Count: 6
Uncorrectable Sector Count: 0
Temperature: 37
Start/Stop Count: 2351
Power-On Hours: 13558
Power Cycle Count: 31
Load Cycle Count: 2334

Device: sdb
Controller: 0
Channel: 1
Model: WDC WD40EFRX-68WT0N0
Serial: WD-WCC4E0HHFNH3
Firmware: 82.00A82
Class: SATA
RPM: 5400
Sectors: 7814037168
Pool: data-0
PoolType: RAID 5
PoolState: 5
PoolHostId: 7c6ee44a
Health data
ATA Error Count: 0
Reallocated Sectors: 0
Reallocation Events: 0
Spin Retry Count: 0
Current Pending Sector Count: 0
Uncorrectable Sector Count: 0
Temperature: 38
Start/Stop Count: 2364
Power-On Hours: 13558
Power Cycle Count: 30
Load Cycle Count: 2352

Device: sdc
Controller: 0
Channel: 2
Model: WDC WD40EFRX-68WT0N0
Serial: WD-WCC4E6FY46HE
Firmware: 82.00A82
Class: SATA
RPM: 5400
Sectors: 7814037168
Pool: data-0
PoolType: RAID 5
PoolState: 5
PoolHostId: 7c6ee44a
Health data
ATA Error Count: 0
Reallocated Sectors: 0
Reallocation Events: 0
Spin Retry Count: 0
Current Pending Sector Count: 0
Uncorrectable Sector Count: 0
Temperature: 39
Start/Stop Count: 2314
Power-On Hours: 12364
Power Cycle Count: 30
Load Cycle Count: 2302

Device: sdd
Controller: 0
Channel: 3
Model: WDC WD40EFRX-68WT0N0
Serial: WD-WCC4E0HHFFKS
Firmware: 82.00A82
Class: SATA
RPM: 5400
Sectors: 7814037168
Pool: data-0
PoolType: RAID 5
PoolState: 5
PoolHostId: 7c6ee44a
Health data
ATA Error Count: 0
Reallocated Sectors: 0
Reallocation Events: 0
Spin Retry Count: 0
Current Pending Sector Count: 4
Uncorrectable Sector Count: 0
Temperature: 37
Start/Stop Count: 2338
Power-On Hours: 12366
Power Cycle Count: 30
Load Cycle Count: 2324