1. If my H1 is approved will they send new I-94 along with that or do i have to apply for that too?

Yes, they should send you new I-94. The number could be same as you have now though.

2. Since I am applying for master's quota. Do you guys by any chance know what is the minimum salary requirement to get approval for h1-b visa?

Look in Onet Wage Library in Department of Labor Web Site. You can google and find this web site.

Wage varies according to location, job levels.

3. And also is it fine to send new I-20 while we apply for h1-b visa to bridge the gap between june and oct?
Whenever you deal with USCIS, it is recommended that you prove you have maintained status.

4. If i send my new I-20 will my OPT expire?
Hmm dont know unless if the I-20 is for different university or different program than your previous fro which OPT was obtained.