You are an absolute legend!!
Thank you so much for this information.
After 6 unsuccessful attempts I was coming close to going through with the OTA update but then I found your suggestion.
Initially I may have received this error because my phone auto locked. Not sure if iTunes then kept looping on this error.
Anyway I followed the trouble shooting for this error : I made sure that I had the latest iTunes, rebooted twice even, I connected iPhone 8 directly to PC (Windows 10), turned off auto lock on my iPhone, closed iTunes, re-plugged the cable... but to no avail.
The Error 4000 kept occurring at the Restore point for some reason.
Log
[04:00:43.0111] Stashbag commit failed: failed to set manifest: -536870212
[04:00:43.0128] Failed to commit stashbag
[04:00:43.0128] Finished BootedOS Restore Phase: Failed
[04:00:43.0128] State Machine Dump, status:ERROR - [state:BootedOS remaining-cycles:0] -> [state:Recovery remaining-cycles:2 (current state)] -> [state:RestoreOS remaining-cycles:1]
[04:00:43.0128] Changing state from 'Restoring' to 'Error'
[04:00:43.0128] Restore completed, status:4000
[04:00:43.0128] Failure Description:
[04:00:43.0128] Depth:0 Code:4037 Error:Failed to commit stashbag
[04:00:43.0128] Depth:1 Code:4000 Error:Stashbag commit failed: failed to set manifest: -536870212
[04:00:43.0128] State is now set to error: Failed to commit stashbag
Anyway I've now successfully updated my iPhone 8 from iOS 11.4.1.
And it only took around 20 minutes to update. It really wasn't long at all.
I would imagine it would take waaaay longer using OTA.
Update: I've tested my apps and force touch etc and everything is sweet. Very smooth. Loving it!!!
Thanks again notCoffee!

You made the update experience a pleasant one for me.