The helium-4 nucleus or alpha particle with a mass of 4 is particularly stable. But there are no is stable isotope with a mass of 5. Helium-5 isn't stable, nor is lithium-5. They decay almost immediately. Lithium-6 however is stable, but it has a lower binding energy than helium-4, which sounds relevant. Something else that sounds relevant is that lithium-7 is stable too but lithium-8 is not, and nor is beryllium-8, or boron-8. The $64,000 question is why?
Why are there no stable isotopes with an atomic mass of 5 or 8?