Are proton, antiproton, electron, positron the only observed subatomic particles that can freely exist and don't decay, i.e. are stable? Are proton, antiproton, electron, positron the only subatomic particles that can freely exist (i.e. I don't want particles that only exist in bound state as constituents such as quarks) and don't decay, i.e. are stable?
What about muons?
Are there other particles/hadrons that can exist freely (i.e. not in some bound states) and don't decay?
 A: No. At a minimum neutrinos and anti-neutrinos are also stable{*} and exist in unbound systems.
Additionally the electromagnetic carrier boson (i.e. the photon) can exist for arbitrarily long times in the reference frames of massive objects (it's proper time is necessarily zero). The same could be said for the graviton if we had experimental confirmation of it's existence.
Further, many beyond-the-standard-model candidate theories feature (anti-)proton decay, though the current experimental limits require this to be a very slow process indeed.

{*} We have to be a little careful about what we mean here. The pure mass states $\nu_i$ are stable in free propagation, however neutrinos are created and destroyed in flavor states. So we have time-dependent superpositions of mass-states, in which the sum of the lepton flavor numbers is conserved.
A: In addition to the other answers, the lightest magnetic monopole is stable by magnetic charge conservation.
A: The lightest super symmetric neutrinos are theorised to be stable, and as such represent a candidate for dark matter.
