Try using the earphones with the microphone attachment and listen if you can hear the caller OK.
Also try using the speakerphone option as well to listen if you can hear the caller.
Both the above are just to find out whether there is a problem with the phone's receiver or if there is a problem with the voice reception circuits on the systemboard.
Whatever the fault may be, since you have performed a factory restore (is this correct?) and the problem still exists it indicates that it is a hardware problem not a software problem.
You state the the phone is only 4 months old. I suggest that you verify the purchase date of the phone to see if the manufacturer's warranty period of 12 months is still valid. If it is, consult the manufacturer's warranty statement (usually found in the User Guide that came with the phone - it may be different for your location to what is shown in the link above) as to what to do to make a claim for a manufacturer's warranty repair or replacement.
If you do send it in for a repair or replacement ensure that you have performed a factory restore as this will mean that all your personal data will have been erased from the phone and it will be in a factory default condition, just like when you first got it - except of course that now it doesn't work properly ;-)