Your employer is liable for flights back to the UK, thats about it. If they wanted to end your contract at any time they could, regardless of if you have applied for PR or not and you would have to leave the country or lodge another visa within 28 days.
Obviously they also have to comply with employment law so no discrimination etc but if it didnt work out or if they no longer needed you they could end the contract.
I suppose flights is something! I was trying to work out if a the end of 4 years and no PR granted etc if my employer would have some sort of duty or obligation to find me a position or role in the UK or if I'd be left to work all that out myself...?
Nope, they just have to pay for flights so you are not in the country illegally. That is unless something is written into your employment contract to say they have to provide more. BUt even then if they go bust and you get made redundant you'll get nothing if they havent got it to give!
Why are you thinking you might not qualify for PR? It should be easier to go from 457 to PR with the new system coming in, not that I fully understand how.