You'll need to contact the retailer for assistance.  Dell ships new systems to retailers with about 18 months of warranty.   If it's expired after 4 months, it means the system was sitting on a shelf with them for about 14 months before they sold it.

It may or may not be.  Look up the original ship date of the system using the service tag number.  It may be quite some time in the past.

You can also try registering the system with the date of purchase, but if the system was indeed more than a year old when you purchased it, you'll need to take up that issue with the seller.  

https://www.dell.com/support/assets-online/us/en/04/registration#/identifyservicetag

I tried registering and they would reply in 10 business days. But there has been no reply since. 

Also when I contacted the retailer they said that the purchase receipt is enough for getting service.