I doubt that they'd merely release season four encased in a box series set. I don't know of any other series of DVDs that does that. That being said, they could release the box set before the single version of series four to stimulate sales of the special edition, but they'll likely come out around the same time. I betcha!
I am an avid DVD collector and don't see HIT doing anything different than other releases. Don't panic. No one will likely have to get the whole box set. If anything the season packaging will be limited edition numbered. That's just a guess.