You've already got an A1200, so why on Earth do you need an A4000? The only things I can think of is that the graphics cards are a little cheaper for the A4000 than the A1200 and the Video Toaster 4000 is useful for video editing.
For anything faster, you'd be better off with either a Pegasos 2 or a non-classic AmigaOS 4-capable system that is yet to come out such as the
SAM440EP.
