From what I've found so far, and from others on here, a lot of the engine stuff on later evo's is cross compatible with the IV.
You might be surpised what you can find locally for USDM cars like the Evo 8. I've gone into NAPA several times and gotten parts for the USDM Montero, (for my JDM Pajero), and I've also gotten them to look up the Evo 8: it shows in their system. Partsource as well...
Current: 1997 Pajero Evolution no.581, 2013 Lancer Sportback, 2003 Montero
Not forgotten: 93 Pajero, 91 Pajero, 96 Evo4, 91 Galant AWD, 89 Montero, 89 Raider, 88 Raider...