I'll assume that your work history with the company is sufficient to qualify you for the L1 visa.
If we are talking about a blanket L1-B visa, you must go back to India to get the visa stamped. If we're talking about a named petition (a L1-B petition specifically in your name) submitted to USCIS for approval, you should be able to change your status from H4 to L1-B from within the U.S., and start work once you receive the approval notice and new I-94.
If it's urgent your employer's immigration law firm should be able to assist.

