The four particles paradox in special relativity

We present a novel paradox in special relativity together with its solution. We call it the four particles paradox. The purpose of this paradox is pedagogical and therefore directed towards students and lecturers of physics. Even if most paradoxes in special relativity theory are very interrelated and some are special cases of others, the paradox we present here is original and illuminates on the very nice subject and the literature of special relativity.