Australia had an unexercised option for a fourth Hobart-class DDG, but that option expired either in 2008 or 2009, during the Rudd gov't. It would have taken significant effort, as well as whole new contracts and approvals before an Abbott gov't would have been able to order any additional Hobart-class destroyers built. Effectively that ship sailed during the first Rudd gov't.
I think the option was somewhat possible until possibly after the second election in 2010 before it was deader than disco. 2008 was too soon after the election to really been a viable option and there were other priorities with the global economic crisis. Certainly lobbying basically stopped post 2010, Rudd was out and Gillard was in and no only was it late to start sourcing things, it defence was scaled back, and the project was in all sorts of build related issues. The 3 hobarts were then further ruined as a project with steps to delay the project deliberately to save some money short term, but cost money long term. The Hobart design was quite stagnant by that stage, being a minimum refresh of the F-105, which itself was a min refresh of the F-104. It may have been a 4th Hobart was never possible. Which is the problem if you choose a "proven" design that isn't actively in a continuous build. Really any 4th Hobart was really looking more and more light a flight II type not an original configuration.
As it was the 3 ships are having their aegis systems replaced almost immediately, and the last 2 ships were built with helicopter modifications. So really it was never a +1 option, as to fund/make viable the further updates, more needed to be built, or its just another white elephant.
Which is I think part of the platform selection issue. We don't build/operate/support a single platform in a vacuum. The program should have been 6 Hobarts to replace 6 FFGs. Six hulls would give us reliable sustainable capability. The ecosystem for crewing, logistics, SME support would have been far more sustainable and efficient. It would have been cheaper than what we actually did.
If we had 6 hobarts today in the water, decommissioning Anzacs wouldn't be a crisis, we would have a useful fleet, even with the first 3 hobarts coming out of the water for upgrades.
Which is why I actually hope Tier 2 is actually a bigger number than the minimum. Should really be a least 9.