Why there isn't any Intel Core i4?

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http://superuser.com – I am planning to buy a newlap and while just searching about the cores, I couldn't find any Intel Core i4. My question is: does the Intel Core i4 exist or am i wrong. If it exist what does it stand for? What is the difference between i4,i5,i7(and i6 if it exist). (HowTos)