This question already has an answer here:
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?