My 4 year old has severe stomach cramps every 2.5hrs and the last time was complaining about back pain.

Depends on severity: Normal, healthy kids can get a stomach flu or food poisoning symptoms from time to time. A patient may have vomiting or diarrhea until his stomach is all empty, which is why it is important to NOT eat or drink anything for a few hours afterwards. After that, a common treatment is small amounts of Pedialyte or broth, maybe 1 ounce every half hour while awake. SEVERE cramps needs a doctor's EXAM.