Even if you had paid insurance, the package was delivered so there could be no insurance claim. Maybe he sent all four and two did fall out, or taken out of bag. Maybe he only put two in thinking he put in all four. Maybe ask him to look around and see if he might have left them out of package. How is his feedback rating? If it is very good then maybe the package did start out with all four. Did it look like the package was open? I would think if a postal employee opened the package and took out two, they would have taken all four.

End result - doesn't look like you have much of a claim. If his feedback is poor, might want to post his name so others can be aware. Up to you.