This problem could have many causes. The most likely are: 1) faulty thermostat; 2) defective (cracked/broken/missing) dip tube; 3) and reverse-installed water heater. Sludge or scale buildup in the tank or fittings could also cause this problem, but seems unlikely given the newness of the tank and plumbing system. I presume you’ve checked to make sure that the valve to the tank is wide open.
Since you say you are unfamiliar with plumbing, I’d advise calling in a plumber to check it out. Since the tank is only 4 years old, it is still under warrantee. I’d also suggest calling the contractor who built the home.
NtP