The chances are very, very low. In terms of the cut-off date to have AWD #4 follow after AWD #3 the key issue is the long lead time required to build the AEGIS combat system. The whole thing, radars and computers, is built and then testing in the middle of New Jersey (The Cruiser in the Cornfield) and then shipped out to the yard. Anyway the cut off date for a #4 was probably last year but you could probably get it built if you long lead order in the next 18 odd months.
You figure the cutoff was last year? Hmm... I was figuring there would still be another 24-36 months that the order could be placed and have the 4th AWD follow the construction of the 3rd with a minimal gap.
My guestimate was based off the 3 radar and computer systems being ordered in Jan. 2008, with work on the construction blocks starting in 2011 and then an in-service date of 2014. After that, the other two hulls following at roughly 18 months intervals IIRC.
Based off the figures for the radar system, it takes ~4 years between the order being placed and the system completed, tested and then available for install from what I can tell.
Assuming that the 4th AWD was ordered to follow HMAS Sydney, I would expect delivery in 2019-2020, with the radar and computers not likely needed or install until about 2018, which is where I got the 2014 date from.
I agree though that the chances of a 4th AWD are really not very good.
-Cheers