My understanding is the four pillars is still in existence today and it ensures the 4 major banks (WBC, CBA, ANZ, NAB) could not merge but could acquire smaller financial institutions. It used to be six pillars from memory as it included AMP and National Mutual as insurers in the past.
Secondly the very nature of the way the four major banks are required to be structured with capital adequacy insulated them from the full force of the credit crisis and subsequent global financial crisis and the government didn't have to afford them any form of assistance or protection.
These four banks dominate the financial landscape in Australia and tend to underpin the Reserve Bank of Australia. Capital adequacy considerations are administered by APRA and APRA is feral in keeping the four majors topped up. There has been debate in the last few years about the four pillars being more a hinderance than a help but it most certainly helped Australia weather the GFC fallout. Many of our ASX entities took the brunt of it including the four major banks.
I don't remember the reverse where the government ever providing a guarantee to the four major banks, what it did do a few years ago was provide a guarantee for depositors up to $250,000 and this was the result of a series of financial institution failures, not one of which were the major banks. (Note: I got corrected on this in an earlier post as I wasn't aware of the $250k protection).
