Insert a usb key with 1.5.1 update img on it (root, and unzipped).
Power on Prime 4 and maintain VIEW button on… It should start updating (that’s what happened to me)
Is there a contact number for denon please UK it’s stuck on load screen after update!
which USB slot?
so… how to force a downgrade/try again when its bricked please?
The first slot, next to the sdcard slot.
Works well. Don’t know but the Prime 4 seems a bit slower in tracks loading.
Except the size font bug (when you switch big/small)it works very well.
found it - power on with eject button held
can confirm issues for me with mac updater - downgrading to 1.4.1 then a usb update did the trick 
Ok, I was able to go back to the previous version (1.4) without any problems. Unfortunately, the error is still there when updating via USB or PC… 
The issue you’re seeing is something that we’re not familiar with. To be honest, this one is a bit of a stumper. I’m going to see what we can do to resolve this and will be reaching out to you over PM if thats ok with you. In the meanwhile, I hope you’re ok with 1.4 for the time being while we figure this out.
Thanks for your understanding,
Vince
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Ok, please contact me.
Format a usb key in Ex-Fat (with a mac /Paragon or a Pc) Copy the 1.5.1 img unzipped file on it (don’t forget to choose the right file : usb file on USB…)
Power the P4 stucked with the VIEW button pished until firmware upgrade begins.
That’s what works on 2 Prime 4 (Mine and another DJ)
On the other hand, no problem with Prime go update.
Are you sure about VIEW? I think it’s EJECT.
With the view button…WHAT?! In the UK we’d say that to describe having rather too much alcohol.
Oh wooow! I haven’t got pished for ages 
. Pushed
For me it was the view button. That’s curious… Or the eject button unbrick a device too
It could be a last resort, if with other methods it is not possible to restart the P4 or when it is locked for some reason. 
Wanted to drop by my feedback as I also experienced my Prime 4 bricking immediately after 1.5.1 Engine OS update. I updated via MacBook Pro with USB cable.
Worked just fine right before update; I tested a few songs and tested my mics as a control test to experience the before and after mic threshold update.
I will attempt to revert back to 1.4 via USB thumb drive as others have suggested. My unit is stuck at the “Engine OS” display screen when turned on. Nothing else happens.
Thanks to @DJDark for providing a tutorial in another post on how to revert your Prime 4 back to 1.4.1 so that is not bricked for now. Fixed without issue. https://community.enginedj.com/t/i-ve-just-updated-my-prime-4-to-1-5-1-now-it-s-stuck-on-load-screen-help/24573/9?u=dj_ramo
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Plus others bugs on Prime Go (play button stuck randomly and the only way is to reboot)
Please 1.5.2 as soon as possible.
Tonight I gig on P4 1.5.1 I hope I won’t have bugs.