Sorry to hear that Neo. can you please advise which supplier you purchased the 4k from? Gearbest? Amazon? Aliexpress etc? even if you did purchase the Pimax 4k back in Dec 2017 that is still only 5 months ago - and your 1st post here was 15 days ago.. so in fact its 4 months ago!
If the unit is defective then they should replace .. usually the warranty is a year but will not cover accidental damage. But the warranty may be dependent on the supplier and what the actual fault is..
if it is down to damage from say being dropped and not a 'hardware fault' (not working as designed - no fault of your own) then im sure Pimax will be able to honor the warranty and can speak to the supplier on your behalf. they may require screenshots of the damage or issue you are experiencing so they can make a judgement call.
for instance.. my headset top strap broke - only a few months into using it.. I contacted Pimax Support, i provided photos and purchase details and supplier details of my order, and pimax then sent me replacement strap and face cushion completely free under warranty. without dealing with gearbest (my supplier).. but they wont have spares of everything and like i said it is down to what the fault actually is!
hope this helps