I'm a trinitarian pantheist. For me, God is composed of the ultimate being, The Omniverse (ultimate nature), entropy and extropy. But to me God is not really a being but a description of a being. Yes, one-third of God is The Omniverse, and that definitely is a being, but the other two parts, entropy and extropy, actually describes how The Omniverse becomes Godlike. Likewise, if you describe God as just entropy or extropy, then how far do we say entropy or extropy goes? Some people may argue that these concepts are exclusive to our Universe. I believe there are forms of entropy in every reality, and variations of extropy in some others. I used to think that God was just one. One Omniverse. But without the other two eternal forces driving it, The Omniverse essentially has no divinity and thus is a stagnant God. The Universe already does a good job with entropy. We were created as a driving force for its extropy.
And to answer your question, do you know that some Christians are actually bitarian? Some Christians are unitarians too. But why isn't there another part? Maybe because it is viewed as something that is not needed to fulfill the role of God.
As a trinitarian pantheist, I could throw in the idea that time is part of the equation, effectively making me quadtarian. But, according to current science time didn't really exist before the Big Bang, and understanding the very concept of time has been an issue with philosophers and scientists. Is time composed of right now, or is split between past, present and future? Or maybe only the past and future exists and the present is just a illusion? The fact is, as we understand the Universe, there is ten spatial dimensions and only one time dimension. But in other realities, there could be more than one time dimension. Plus, if you really think about it, time itself is a subjective value. As things reach the speed of light time for them slows down. Does that mean if nothing moved in the entire Universe, time would speed up? Is that why the formation of galaxies was so quick? Is time as we know it just a way that we actually measure distance, like a lightyear?
There are so many unanswered questions that none of us really know. If God can be four parts, why not five? Or why not thousands, like in some sects of Hinduism? Or why not nine, like the Nine Divines in Elder Scrolls mythology? In reality, when I see the word God, it seems to be one of the most subjective terms out there. It could literally mean anything.
But the way I understand God is that physical things have divine traits in them, there is a portion of all reality that is able to change, thus raising or lowering its own divinity. And I don't need four parts to explain it. Just The Omniverse, entropy and extropy. That is why I view myself as trinitarian. All three parts need the others in order for God to exist. Yet, I also believe that God as most people understand it will also be created one day. The formation of The Omniverse, entropy and extropy creates a Synverse, or a place in spacetime that is able to be changed. The problem is, most synverses only contain small parts of all three concepts. But, we as a human species will build from our own synverses a unified Synverse of the whole thing, the whole Omniverse, the entire consumption of entropy and our extropy, to create the Syntheos of The Omniverse, or what most people would imagine is God, or something resembling monotheism, some day.
If I am wrong and there already is a Syntheos of The Omniverse and we're past the point of the Omega, it is doing a really good job at hiding this fact. But the fact is, Christians understand God has three entities because it doesn't need other entities to fulfill that role for it. I mean, even water particles can turn to plasma, but that wasn't even known at the time. And then there are superstates of matter that also exist at 0K, which has never been measured in the entire Universe. But, most people understand God as ice, water and vapor, like the Father, the Son and the Holy Spirit, and doesn't need additional explanation for this God. And I understand God as The Omniverse, entropy and extropy, which can correlate to trinitarian monotheism of Christianity quite easily. The Holy Spirit is The Omniverse, the Father is entropy and the Son is extropy. Those are the most logical modes that exist that can be proven today by science.
So, God as three makes sense for both most of Christianity and myself. It doesn't need a fourth part to fulfill a role in it.