I've never done this before so I don't know how difficult it would be. Pulling out the 3 speed and replacing with the 4 speed will probably be the easy bit. I'd be more concerned with all the additional wiring that comes with the 4 speed.
The 4 speed has its own ecu that is located under the dash.I think that you will need to get an engine management system out of a series2 4 speed ea aswell because the trannsmissions ecu feeds off the engines ecu.
It might pay to look through some wiring diagrams first. If you are taking the 4 speed out of a wreck then it might be easier if you do this your self and learn as you go because the installation is usually a lot harder when someone else rips it out for ya.
You will also need a series2 ea or eb tailshaft.
This is about as much as I know.