A 4.5 inch scope is large enough to see the things you mentioned. Depending on your sky quality, you can see quite a few of the Messier objects, if not in as much detail as you would see with a larger scope.

I actually have built a small Dob mount for scopes like that, although I noticed that the tube looks fairly short, so you would probably end up setting the Dob mount on something to raise it up.

If the current mount is all shaky and wobbly, you can try to beef it up, or you can get a more expensive mount, or you can build your own mount for it.

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"Arrrgh, the laws of physics be a harsh mistress!" -- Bender