Yes, they're grammatically correct, but usually used only in dialogue, not in formal writing.
3 is fine, but 4 is not. You don't need "the same" at the end. You can either keep or omit "is" before that.
In 3 and 4, "as" is being used as a conjunction.
http://www.dictionary.com/browse/as?s=t
1. Jenny's friends're not angry.
2. Your friends're generous.
(Are the contractions grammatical?)
3. Middle school is not the same as elementary school.
4. Middle school is not the same as elementary school is the same.
(Does #3 mean #4? Is #4 the full form of #3? What is the part of speech of 'as' in #3?)
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