H4 to H1 Visa transfer

[ngade]

I am currently in US on an H4 Visa. My H1 visa is being processed. Due to unavoidable circumstances, I have to go to my home country in the month of september and i shall be back in october. However, the problem i am facing is that my H1B Visa may or may not have been processed when i am returning back. If it is not processed, things are fine. But if the visa is processed ,Am i allowed to still return on my H4 visa or will i have to get my H1 visa stamped on my passport before i come to US.
I would appreciate if anyone could help me with this.