contraction

Hello,

As I came to know that the adverb "yet" placed either the end of the sentence or just after the word "not". But I have some doubt.

1. I havn't finished my work yet.

2. I have not yet finished my work.

3. Have you taken your lunch? No, Not Yet.

I want to know that when we use contracted form like "havn,t", can we place yet after it , or it is required to write in two seprate word like " have not".

4. I havn't yet finished my work.

Is writting in contraction form in exam correct?

Please tell me the correct position of yet in a sentence.

Thanks.