H1B transfer? effective Date? Help

I got an offer from company A in April and got H1B in June. My OPT expired in May. So I did not work after May. Now, I probably will get a better offer from company B in Oct sometime.
1. I wonder if I can transfer my H1B from A to B?
2. Will there be a problem that I only worked in A for a couple of weeks before H1B transfer?
3. what is the minimum time I have to stay with A before H1B can be transfered?
4. Is it possible I did not work for company A at all and directly transfer H1B to company B?

thanks a lot