Because I have no experience with Snow Leopard whatsoever, allow me to state the obvious and ask if you've installed Rosetta. Myst IV is a PPC only binary and has no native support for Intel Macs. Snow Leopard makes use of Intel binaries only, and did away with anything involving the PPC architecture. This made it faster and smaller, but you'll have to find Rosetta on Apple's web site and install it manually if you haven't already.
The only other thing you can do is install Windows with Bootcamp or Parallels/VMware Fusion and run Myst IV from there. If it's Windows 7, though, it's possible that you'll run into even more issues; I haven't heard about Myst IV being ran on 7 yet. :/