If I recall correctly from my working years, any 3 of your RAID5 drives have all the info needed to rebuild a failed 4th drive. If your hardware is hot swappable, you just unplug the bad drive & plug in the replacement. If not, power it down, replace the drive, & turn it all back on. You might be prompted to recover the data onto the new drive. It takes a while, but the new drive will be rebuilt as a participating member of the array.
From your pics, it looks like a WD drive system. Might be good to look at their documentation for drive replacement procedures.
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