That's a good question. The threads importer goes pretty fast, but 4.5 million posts is huge so it's hard to even fathom. I'm reluctant to even give a ballpark figure as with that amount of posts I might be way off
On a fairly fast server it will do about 15,000 topics per hour, that's including all of the replies. So it somewhat depends on how many topics you have, not just posts.
The way it's currently written is it does 20 topics per refresh. If it's your own server you might be able to modify the script to do a bit more per refresh that could speed it up a bit.