Ok... let's shift gears, then. What is the farthest observable star within the galaxy? Now, we're staying inside the main structure. From my Bortle 7 backyard, I can usually pick up about 10.5 mag.
10.5 is pretty dismal for a 4-inch refractor! You should be able to go considerably fainter than that, even from Bortle-7 skies. High magnification and averted vision.
Off the top of my head, a 4-inch refractor under pristine skies should be able to detect stars roughly to mag 13.0-13.5, depending on the observer. Yet another quick mental calculation tells me that should show supergiant stars out to a distance of at least 100,000 light-years -- flat across the disk of the Milky Way.
Alas, the problem is that supergiants are necessarily young, and all of the Milky Way's star-forming regions lie close to its plane, where the views end up being blocked by dust in almost all directions. The parts of the distant Milky Way that are readily visible, including globular clusters and the central bulge, consist entirely of old, and therefore much fainter, stars.