Visa: H1B
US Arrival Date - 7/31/2004
H1B Petition valid until - 7/30/2010

I work for a consulting company where being on project keeps me on payroll. My current project is expected to end anytime and finding new project is difficult. Can I change my status to H4 (say by 5/31/2009) (My husband is working on H1) and can I change the status back to H1 later on finding a new employer / project.
In this case while on H4 will I be preserving remaining year on H1B that I currently have? I understand it will not be counted in the cap. Also would I need to get back on H1B before 7/30/2010 or once I get on H4 then 7/30/2010 would have no significance and I can get back on H1 even after 7/30/2010?