Interesting, I had something slightly different in mind. I was thinking he'd be quite well attuned to the physical world and so would be able to hold a physical form more-so than most others in 4D STS. My thinking is that he must be quite advanced in his 4D STS path and so the worship of physicality.
My understanding of the transition to 4D and how it progresses is that it moves from more physical to less physical. As one becomes less bound by physicality, one gains more powers to manipulate the surrounding physical environment. A 4D STS entity who has less physicality is freer to act and less subject to physical laws, making things such as quasi-immortality a legitimate possibility. Also, we know that the higher level 4D STS are deeply psychic, very aware of souls, know how to abduct them, interdict them, and artificially incarnate them into other bodies, so they are not ignorant of what we consider to be spiritual matters. Their understanding of materiality is radically different from our scientist understanding, and I've always thought that the hardcore materialism supported by science is more of a straightjacket encouraged by 4D STS to prevent humans from becoming troublesome by discovering the much broader materialism that 4D STS operates within, and is not necessarily reflective of the 4D STS understanding itself. They do however, eventually hit a wall based on how they relate others. In a purely spiritual universe, there is no time and no space, everyone and everything is amalgamated into the unity of the eternal now. Furthermore, in this environment there can be no overseer, all is all. This is actually an STO environment in which the STS polarity cannot function, and so the STS beings need some degree of physicality to maintain the illusion of separation and the status quo between the ruler and the ruled. I think this is why the Cassiopaens said they worship the physical universe and undergo a sort of existential collapse when they gain too much knowledge and try to maintain this contradiction beyond its practical limits.
So it is not that the emperor has trouble maintaining a physical form, it's that it introduces a degree of limitation and forces him to focus his awareness through a very narrow window, like zooming in on something far in the distance through a rifle scope at 50x magnification. The more power he can pack into his physical projection, the wider his field of view will be at that level of magnification, but everything has its limits, and it makes his body behave increasingly like energy instead of matter. On the other hand, his true form may be a field of highly concentrated energy, maybe something sort of like a sentient black hole, but the intensity of his emanations make it impossible for all but a few of his most powerful servants to stand in his presence when he is operating at full strength. At the grand scale on which he operates he is rather esoteric to most of his minions, who periodically get uppity and question their obedience to such far-fetched things so far beyond their normal understanding, sometimes going off on their own and interfering with his plans. In that case, a powerful humanoid figure glowing with unimaginable energy can demonstrate authority by making a mountain explode with the flick of a wrist, making their understanding much more...succinct. He probably does sit on some grand throne somewhere part of the time when interacting with his minions, but the part that most of them see is a lesser part used for the convenience of communicating.
Also, I should probably be careful with how I play with the terms physical, energy, matter, and nonmaterial. Energy is certainly part of the physical universe, it is just extremely amorphous and malleable if one can direct it as opposed to matter, which is more fixed and rigid. Both energy and matter are material and physical. Nonmaterial would be something outside of either of those things, pure consciousness basically. So I should say that the emperor prefers the energetic existence to one of rigid matter, and this gives him more power because he is closer to the realm of pure thought where all that is and is not can be conceived, but he is never quite able to reach it entirely.
Where I differ from Gene Roddenberry's depiction of "the big bad" is that I don't believe he is older than the physical universe and has been around since the beginning with a singular coherent lord of darkness identity. The real emperor may be able to travel to the beginning and end of time of the physical universe as we understand it, but he arose within it and is somewhat bounded by it. We would certainly perceive him as quite ancient, but ultimately he is born, rises to power, falls from power, and dies like any other king.
] in the series.
to all.