Some employers "force" their employees to use a portion of their annual leave during the Christmas / New Year shutdown period (usually 24 December -> first full week after New Years Day, if not longer). So you might not be able to use the full 4 weeks continuously.

This can be an unwelcome feature for some people, for example, if you want to have a vacation in the northern hemisphere summer season instead and/or maybe you don't have substantial family in Australia (or at least, those you actually want to see).

The auscorp reddit has a yearly thread on this issue: https://www.reddit.com/r/auscorp/comments/1mw6pqt/end_of_yea...

Those with school aged children might also want to save some of their annual for the mid-term/mid-year breaks as well. (Our academic years are aligned to calendar years)

Likely varies by industry - a peer Australian (probably in private IT ?) stated it's uncommon to take a break, whereas I'd say in mining, oil, gas, civil service, police and a good number of structured contract employment its more common.

I've "retired" into agriculture and a lot of farmers take a month off after harvest time to go fishing or other wise relax (this generally means filling up a couple of deep chest freezers with fish for the rest of the year).