Mike, what are the "most purposes" of which you speak? On all but the brightest deep sky objects an 8" reflector will walk all over a 4" refractor. Not saying the 4" frac wouldn't be nice but when I had one at the same time as my C8 it was the C8 that got used almost all the time.
Most purposes for me are mainly lunar and planetary, but I also enjoy using my 100mm refractor for deep sky too. I do recognise the advantages larger apertures provide, but I don't know what it is exactly that makes me enjoy my 100mm more than the 250mm F6 Dob that mostly stands unused in the corner of my observatory. I love the sharpness and purity of the views through my 100mm, and the definition it provides is simply glorious. The views and level of detail in my 100mm is jawdroppingly impressive when it comes to targets like Jupiter, Saturn and Mars. Even Venus is swathed with subtle cloud top detail. May be the Dob is just too bright for my eyes, while in the 250mm, although I can certainly see the detail, its just not as well defined.
As regards the challenge of fainter deep sky targets, perhaps it's the challenge that I enjoy. So far the most difficult target I've successfully observed with the 100mm is IC 474, and a tiny black notch in the ghostly glow that I believe was the horse head, although I didn't see any shape to it. Brighter DSO's can be an absolute joy. I've attached a sketch I made some time ago of M27. Most people see it as a smudge, but that's because they don't take the time needed to really study it. Perhaps that's the key to getting the most out of a small aperture scope?
Unfortunately, or fortunately depending on your view point, I can only post the one image at a time. Otherwise I could bombard you with some beautiful deep sky sketches, each one taking 30 to 60 minutes to observe properly.
Stellar wise, the 100mm will reach an easy mag 13.2 and remains sharp on a night of steady seeing up to 1000X on double stars. To be honest though, although 1000X is a nice party trick, there is no meaningful advantage over 500X, so I don't use 1000X with any serious intent. It's just nice to know the stars are textbook perfect even at rediculous magnifications.
Of course the planet's put on quite a show even at medium powers. The attached Mars sketch from last year was made using a binoviewer and a magnification of approx 180X
When it comes to the Moon, the 100mm could keep me occupied for several lifetimes. It resolves the central rille running along the length of the Alpine Valley floor nearly every lunation. And ultra fine rilles on the floor and up the terracing of the crater Werner that most seasoned lunar enthusiasts are blissfully unaware even exist, or can't even see in their larger apertures.
So as you can see I'm pretty content with just a 100mm refractor, and not as content with the larger Dob's. Perhaps my UK skies have a lot to do with my lack of contentment with larger apertures. Although the 8.5" refractor at my local astro club has no difficulty whopping the reflectors no matter how big they get. But the SCT's just never make the grade here. They can't produce anything even approaching a textbook star image and so always give soft, lacklustre views. Again, it most likely our skies!