The pup is 4 months ...since yeterday when my sister bought him .....the person that sold him sold him like that and today he told us to "take him to the vet ASAP"...but why should i take him if he brought him over like that .....please help telll me what could i do?

I don't want to speak out of turn here, but, you should take him because he is now your responsibility.
Deal with the person you got him from later.
A pup that small can't stay sick for long without things potentially going downhill fast.
A couple of questions though"
Do you know if he has had any vaccines?
Did you change his food suddenly?
Are you noticing any vomiting or diarrhea?
Is he lethargic or depressed?
If you lift his lip and look at his gums what color are they?
A vet is your best bet, there could be a lot of things going on here.
From intestinal worms (parasites) to low blood sugar to genetic problems.

To put it bluntly: Take him back to the person who sold him and let him take care of the sick pup or take the dog to the vet.

It the person who sold the dog is telling you to take him to the vet ASAP, then he already knows he sold a sick animal. More than likely he has more sick pups on his hands or has sold them to other people. Hopefully, he didn't have an as is contract that makes you solely responsible for the animal. If he won't take the dog back, then you either take the dog to the vet or to your local shelter and surrender him to someone who will get him the care he needs.

Your sister got scammed.

You will have to take the dog to the vet yourself I'm afraid. Any new puppies should be vet checked when purchased anyway.