I know these work on OS X 10.5. You'll have to check for compatibility with 10.4
The most reliable, but most expensive: A Windows virtual machine using Parallels or VMWare Fusion. $80 plust the cost of a Windows license.
Much cheaper and mostly reliable: $40 Crossover, with IE 6. They say you won't get full operability this way. I used it for the last week and it seems OK. I can get Active-X controls, and even an embedded Citrix client.
Free, but not recommended: IES4OSX From their own website: "known issues: Internet Explorer uses up to 100% CPU", and sure enough that's exactly what I saw. Sheesh. Oh well, maybe next version.
Edit: Oh, and bootcamp which is free but you still need a Windows license.