4K is 3840 × 2160.
I don’t know about 1440p and TVs. TVs seem to behave in ways that many bespoke displays do not. I have not educated myself enough to be able to explain or answer why that is. I’m sure someone more knowledgeable than me will chime in here.
4K with a Series X is a big deal and completely worth it. In many ways, you’re shortchanging yourself on the entire point of the step up from the earlier Xbox One. The biggest improvements with the Series X is 4K and fast loading times. You’re only getting the latter with a 1080p display.
I see a massive difference in visual fidelity when I use the Xbox on 4K vs. the PC on 1080p. The cockpit detail with labels and instrumentation is a major upgrade on 4K.
Regarding the 60Hz vs. 120Hz. If you’re only using the Xbox for MSFS, then the 120Hz is really not something that is essential or needed. My 28" 4K display is 60Hz. I have another Series X connected to a 55" 4K 120Hz display that is used for gaming, and it makes better use of that 120Hz. The Series X connected to the 60Hz display only has MSFS installed on it. MSFS doesn’t really benefit from the kinds of framerate boosts that a fast-paced shooter-type game does.