Recently my 4yr.old started getting pains behind the knee, any idea why?

Knee pain: Growing pains are fairly common, especially if they alternate legs. Osgood sclatter disease is usually causing pain in the front of the knee. Not dangerous but can be painful. I suggest you take him to the pediatrician, especially if the pain is affecting his ability to play, run, sleep etc.

"Growing Pains": Kids don't do warm-up stretching before they run and play, so they may get minor trauma to their calf muscles. Inflammation builds up over several hours, so the pain may wake them at night. Some reassurance, calf massage, and gentle heat--with Acetaminophen if severe--will make them more comfortable. Ps-growing is not painful, and this type of pain is reassuring that they're getting exercise!