Can i enter at a juris doctor program if I graduated from a UK university?
I saw at some sites that the universities want a 4 year USA degree
FYI, the English unis typically have 3-year degrees but the Scottish ones have 4-year degrees.
Can you point to those sites you saw?
Yes, you can. There may be exceptions, but most law schools want an undergraduate degree from an accredited college or university. I know 3 UK students with undergrad from Durham, Oxford, and Edinburgh who are currently doing just that.
http://www.internationalstudent.com/study-law/
there is also at Yahoo answers but can’t find it now 
The question is, why would you want to do that?
Why not? What is the difference between getting an undergrad degree in the US followed by a law degree in the US vs doing the undergrad degree in the UK and law in the US. You end up in the same place in the end.