My experience doesn't exactly mirror your question, so take it for what it's worth;
I have a XD 9 sub, which I like a lot. However, I've been taking a number of pistol classes and participating in organized shoots involving shots at some distance, and thought that a full size might well be better suited to these activities. I fully intended on picking up a 4", but lucked into a used 5" 9mm at a great price. I picked it up, and absolutely love it. It's a lot more accurate than the 3" sub and easier to shoot well. The extra inch makes no real difference in carry, and the longer sight radius really helps with accurate shooting.
If i were in your position and could get a 5" currently, I'd go with it over the 4". However, like you point out, the 4" are around now and the 5" are currently just vaporware. Nevertheless, in your shoes, I'd wait. IMHO, YMMV, etc.