I have backup mic, headphones, music, cables & so much more BUT what happens if the Prime 4 totally packs in when performing at a live event? The answer will probably involve apologising, packing up & quietly sneaking off in embarrassment.
I know laptop Djs will have backup options such as phones, tablets, mixers etc.
Have you a contingency plan in the event of your worst nightmare?
No matter how well prepared you are, you can’t have a backup for every scenario…
What if the main fuse blows and no one there has access to the fuse board? The line has to be drawn somewhere 
OTOH my suggestion would be - take a laptop (containing music library and DJ software) and a small mixer/controller (I have a Reloop Mixtour).
Totally agree that you can’t plan for everything. I thought about carrying a laptop & a cheap mixer but decided against it. I have so much stuff to lug & set up as it is and I have accepted that if the P4 just packed in, I’d just apologise & obviously wouldn’t charge for the night. I don’t think the client would be totally mad because we all know that sh*t can happen.
Don’t we all? 
As this is the case though, a laptop and small backup player aren’t going to add much to the load.
I’d regard being able to play music as the most important aspect of the gig, so would certainly be trying my best not to have to cancel totally. A backup of that aspect is the most essential IMO.
I suppose the most convenient back up by far is the Prime Go.
No fuss of swapping out XLR to RCA or booting up a laptop & connecting a mixer etc. Literally pull out from P4 into the Go, add your SD & you’re off again. No having to maintain additional dj software (despite having Serato Pro, I literally uninstalled & dropped it - along with my beginner SB3).
Prime go is a good solution for backup device. Fully agree, if cost is not a problem, comparing with a cheap controller as a backup.
A controller, a laptop, dj software license… this backup can overgrow the prime go very fast actually…
iPhone + djay pro + auto mix