Years ago I learned a term when it comes to rusting.
Perforation, easily explained as rust penetrating from behind the paintwork and causing bubbling. This was on all 4 wheel arches of a Ford Orion that was 5 years old, from almost new (ex demo) and low mileage. The warranty was refused on age. It seems that the salt on UK roads can cause rapid deterioration of your chassis and body work and no warranty will accept this as being a factory fault nor normal wear and tear.
However, as has been suggested, the same will apply if your paintwork has been chipped and you failed to "touch up" the exposed bodywork or undercoat.
At 4 years old a Jeep should not be rusting to the extent that you are describing, in that, I agree with Jeff. Marek I also agree with and as he asked, supply the pictures for further detailed analysis.