Why would you then train him at all? You got your 4 weeks notice, go to work, throw them a manual and let them figure it out. If they complain, say "he doesn't understand me very well".
We were to build a series of similar projects with a turn around of about 5 months each. We built a framework as we went along keeping as much reusable as possible and knocked that time down to around 2 months. Company had a bad year and all the IT and software development was outsourced all over.
That said the guys who came in to replace us were pretty nice. I'm not the kinda guy to screw over others because some sociopath in management screwed me over. Just did the time and left and had to take a pay cut to jump fields (was a pretty specific field) but 3 years on I'm glad of it. If I had stayed it would have been a career dead end for me and things are pretty good now.
Sure it wasn't a great moment in my career but we all get to move on.