In PF2, it's easier to rebuild a creature than to convert w/ equations. And doing so will avoid flukes where the systems don't mesh.
Simply figure out how effective the monsters were compared to other monsters at the same level (or more accurately, how effective you now want them to be compared to their peers), then reference the GMG guide to find a number which suits that competency for that statistic. Remember to balance strengths with flaws because a creature w/ all strong stats really is just a higher level threat pretending to be a lower level threat.
Once the charts are open in front of you, the rest is like clockwork.
This is mostly because PF2 isn't a bottom up system (like 3.X monsters, and PCs in all editions). Final numbers are all that matters, not how you got there nor the underlying math. NPCs usually have many nonspecific bonuses that come from nowhere so that final numbers are where they should be.
Special abilities may be an exception, as in not so simple. Nearly all creatures should have a trick, a specific action choice that makes them distinct, even if they didn't before. And if they did before, it may be hard to duplicate. Paizo gives plenty of rules of thumb and examples can be nabbed from the Bestiary for help like getting a sense of appropriate effects for a level. Even PC races & unique NPCs will have these unique tricks that might be unavailable via standard classes.
I see these as creative opportunities myself.
I have seen someone using an algorithm to convert monsters to PF2.
I left a review, and suffice to say it was a great initiative which however I would not be spending real world money on.
As for 4e monsters specifically, I think there's potential there, as several of 4e's monsters had a lot of unique active abilities. Pilfering and readapting can be done to implement some versions of those activities as PF2 monster abilities. Keeping a bestiary handy (if you have one) can help monstercrafting.