Diarrhoea in almost 4 yr old - when to go to GP?

[Chopsypie]

As the title says really. DS is almost 4 an has had diarrhoea for 4 days. Some stomach pain as well, but no sickness.
He is eating some food, but not as much as usual. He is drinking plenty.
No other problems or symptoms.
When do I start to worry?
TIA

One way to test for dehydration is to pinch up (not to hurt, more to scrunch IYSWIM) and see how long the skin stays wrinkled. If it goes more or less back to flat straight away, not dehydrated.

HTH