Getting an A4000 with a PPC card then a gfx card/sound/network etc. ends up being the same as an "NG PPC Amiga", only more expensive and slower. It's cool to have both on the same machine but I've found it better to choose a classic system to keep for older retro stuff and an NG machine for the rest.
Even if you go the hard way and spend all the money upgrading the A4000 you'll end up reaching this very same conclusion.