I had mine replaced for an EF.
Basically, I got a "Shift Kit", which purely means very firm shifts from the Auto Box. (Kinda like short changing on a manual)
All that was involved was changing the S5 solenoid,, and a change of oil, and filters was thrown in, as part of the Deal.
Anyway, to the point that you are interested in.
It cost me $450 to do (Unfortunately, in NSW-Blacktown area).
I guess call around, and see how much an 4 Speed Auto Box shift kit would cost, as its pretty much the same deal as what you are trying to get.
Hope this helps,