If we had a subatomic particle with four quarks, specifically two up quarks and two down quarks, it would not be a stable particle according to our current understanding of particle physics. The reason is that quarks are bound together by the strong nuclear force, which is mediated by particles called gluons. The strong force is responsible for holding quarks together to form stable particles such as protons and neutrons.
In the framework of the standard model of particle physics, which describes the known elementary particles and their interactions, quarks combine in groups of three to form stable particles known as baryons. For example, a proton consists of two up quarks and one down quark, while a neutron comprises two down quarks and one up quark. These combinations are stable because they have the lowest energy state and are allowed by the laws of quantum chromodynamics (QCD), the theory governing the strong interaction.
If you were to combine two up quarks and two down quarks, you would have a total of four quarks, which cannot form a stable baryon according to the rules of QCD. The excess quarks would seek to rearrange themselves into more stable combinations, such as two baryons or other mesons, through the exchange of gluons. The resulting particles would be a combination of quarks and anti-quarks, striving to achieve a lower energy state.
It's worth noting that exotic forms of matter, such as tetraquarks (particles consisting of four quarks), have been postulated and even observed experimentally in recent years. However, these tetraquarks typically consist of different quark flavors, not two up quarks and two down quarks. The study of such exotic particles is an active area of research in particle physics, and our understanding continues to evolve as new experimental evidence emerges.