The discussion centers on the absence of a hydrogen isotope with an atomic weight of four, specifically questioning the stability of a nucleus composed of one proton and three neutrons. It is noted that tritium, a hydrogen isotope with one proton and two neutrons, is radioactive and decays, suggesting that a hypothetical H^4 would likely be unstable due to internuclear forces. Participants explore the concept of isospin to understand why such a nucleus cannot exist, with one member proposing calculations to demonstrate unreasonable results. The conversation highlights the contrast between the stability of helium-4 and the non-existence of hydrogen-4, raising questions about the underlying principles governing nuclear stability. The discussion concludes with a consensus on the complexities of nuclear forces and isospin in explaining this phenomenon.