Extending a 457 visa and getting a PR in Australia?

I am currently on a 457 Visa and not sure whether my sponsor will extend the same. I would like to get this extended say maybe to around 4 years and ultimately use the time to get a PR here in Australia. Can I still search for jobs when I am on a 457 visa sponsored by one employer and what happens if I get a suitable job with a different employer with a 4-year contract. How does the formalities play out ? Does the old visa get cancelled and will I be issued a fresh visa?