Carl Shusterman: Section 105 of the American Competitiveness in the 21st Century Act is the section of the law which creates H-1B portability.
This section does not restrict H-1B portability to persons who are currently in H-1B status. It is enough that you are in lawful (e.g., H-4) status and you formerly were in H-1B status. Therefore, to answer your question directly, even though your application for a change of status from H-4 remains pending, your new prospective employer may ask the INS to change your status back to H-1B and you can take advantage of the H-1B portability provisions and start working with the new employer as soon as the H-1B petition is filed with the INS.
However, the INS, in a memorandum dated June 19, 2001, states that they believe that Congress did not mean to extend portability benefits to anyone "who has ever held H-1B status, no matter how long ago." The INS indicates that the regulations will establish "some reasonable period of time, such as 60 days after leaving the initial H-1B employer" during which you can utilize H-1B portability. I do not believe that such a limitation is authorized by the statute. However, if the INS imposes a 60 day limitation on the use of H-1B portability in the regulations, you will be taking a risk to try to use H-1B portability if you have not been working in H-1B status for over 60 days. At some point such a provision in the regulations will have to be tested in federal court