What is the difference between "same as" and "same with"?
1. I'm the same as you.
2. (X) I'm the same with you.
3. My opinion is the same as yours.
4. (X) My opinion is the same with yours.

In my opinion, 2 & 4 are not correct, but I've seen many sentences using "the same with.."

Question

What is the difference between "same as" and "same with"?
1. I'm the same as you. 
2. (X) I'm the same with you.
3. My opinion is the same as yours.
4. (X) My opinion is the same with yours.

In my opinion, 2 & 4 are not correct, but I've seen many sentences using "the same with.."

Answer

2 and 4 are indeed not correct. However, "same with" is used meaning "the same thing happened/is happening with me" and probably it got shortened to "same with". You can use it when you want to show that you are in the same situation with someone else, or you have the same habits.
E.g. 
"It's so difficult to start my day without a cup of coffee!"  
"Yeah, same with me - I always make sure I have time for one!"