My 4 month old has a shiny red rash in the folds of his neck and i'm not sure why? he is chubby and sweaty a lot...

Intertrigo: There are diseases with results of this redness. The main one is called intertrigo, and is a macerative effect. By that i mean that the the skin swells almost like dishpan hands. When it takes up enough water then it starts to slough and get irritated. We used to say that the best cure for this is to "hug an air-conditioner."! keeping cool does help but seeing a dermatologist may be necessary.