Is it correct to introduce yourself as "我是 name" Can I say "我是 john?" (wo3 shi4 john). 

Would it be grammatically correct?
From a grammatical point of view your sentence is correct. The pattern 我是 + name is a casual way of introducing yourself. Personally I heard that a lot.
However, in formal situations using the pattern 我叫 + name is probably the best option.

Like user3a said, I think it's true that the structure 我是 + name implies that the focus is on 我, whereas in a sentence experessed in the form 我叫 + name it is your name that receives the focus.