Dell does not sell computers on Amazon. You purchased the computer from a reseller that might or might not be "fulfilled by Amazon" which just means Amazon ships and stores item for a seller. A listing for a Dell computer on Amazon will have the info about the seller on the sales page. It will say "Ships from and sold by ..seller's name......." Make sure to save that seller's name and info and check for warranty info on the sales page. You need to notify Dell that you are the new owner to transfer the warranty even if it is only 4 months. See these instructions--
Contact the reseller and Amazon with complaints or requests for returns. Check out Amazon's return/complaint policy. Problems with the sale must be made within 30 days. Remember that Amazon is a Marketplace now just like Ebay and other sales sites. Buyers should always check the reviews for any seller before buying anything.