You *have* got the Drexler patch installed haven't you? If you don't RCT1 *knows* if you've been using trainers/editors and refuses to load the Sv4 files. The Drexler patch removes this limitation.
So... is Drexler patch the only way? I'm guessing yes. But the thing is, does the patch only make changes to the save files that would otherwise show the "invalid error" thing? Let's say I installed the patch and I make some other save file. Will I be able to open that file without the patch?
Is there anywhere I can find out more about this patch? Ah... I've never really used trainers/patches, ever, in my life, so, yeah, I'm a little dumb on this subject.