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How do I go about either purchasing or licensing an invention that has an abandoned PPA?
If the inventor is already selling said invention but never filed a patent, only a PPA, which expired 4 years ago....do I need to get permission from the original inventor to make, sell or license my own idea, if it is similar to theirs?
What if it is slightly different than the original invention? Same function, different materials or different design?
Only granted patents allow the owner to exert any rights from the patent (backwards in time possibly until the filing date!).
Only pending applications (and revived ones if abandoned recently, but that's a different topic) can become granted patents.
A not recently abandoned application will never give the owner any rights. However, there might be other patents that are relevant, from other parties or for example continuations or divisionals.
As to infringement, if you implement all steps/characteristics of any claim, it doesn't matter what else you do, you're infringing. If you don't but are very similar, ask an attorney, there's a lot of potential traps there.
As a word of caution towards you're other question (I can't answer it right now from my phone because the answer is rather long) - ppas aren't published if no non-prov. Is filed. So there is or was a non provisional application.
