But wait - isn't it also possible that these other worlds don't exist? So does that possibility - meta-possibility - "exist" somewhere too?

This kind of thing "It is possible modal realism is false." is a nice example of a Godel-type statement that doesn't involve math.

I won't have to think about it or research it, because if it is possibly a cure for cancer, then it is actually a cure for cancer.

I'm not particularly familiar with Tegmark's position (beyond looking at the website) but modal realism (which afaict is at least a pretty similar position) just declares "actual" to be a kind of indexical, like "here" or "now". Saying "we actually have a cure for cancer" is like saying "We presently have a cure for cancer." So modal realism (and I don't see why Tegmark IV couldn't do the same) doesn't negate the difference in meaning, rather it just interprets words that we're already confused about.

I'm similarly suspicious of the strategy as well (see the reply to Jordan I'm about to write). But I don't think the argument is dedicated to the negation of the difference between possible and actual.