Delay in going back to US after visa stamping.

Hi,
This is regarding the delay in going back to US after visa stamping.

I am currently in india and I have my return ticket booked to US. Unfortunately there was a delay in receving my H1B petition/scheduling appointment/etc. and I may not be able to return on time. I am worried the client may fire me if I am late. I am still an employee of the company filing my petition. I also have my previous H4 visa stamp valid. If I am let go, can I still return on my H1b visa? What happens at port of entry?