The 4 years period is the four years ending on the day that you make the visa application - so if you apply today, the 4 year period starts 14th October 2001 - so you have to have worked in a send occupation for at least 3 of those years - so it allows for up to a year off for maternity leave, leave withoit pay, gaps between jobs etc etc.
buttuming your IT masters meets the requirements of Group B ACS guidelines, you will need 4 years recognised IT experience before ACS will buttess you positively - so you can not look at the visa requirements until you have met the Skills buttessing body requirements. This buttumes you do not have an Australian Masters where the ACS and DIMIA can exempt you from usual experience requirements.
I buttume you are talking about applying for a 136 visa which is an off- shore visa - you can be in Australia on other temporary visas and apply for the 136 but you will have to leave Australia before the expiry of any visa held (so there is no bridging visa available if you apply for an off-shore visa) and you would also have to be outside Australia before they could grant the 136 visa.
Tony
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