amateur observing - What can be seen with a 4.5" telescope

The aperture of your 4.5" telescope is one thing, it's also important what focal length you have. Is it a f/5 or rather a f/8? The f/8 would be suitable for viewing the Moon, Jupiter, Saturn, maybe even Mars and Venus. You can also buy a good solar filter, attach it in the front of the optical tube assembly, and view the sun. But be careful with that, and inform yourself beforehand! Your eyesight might be in danger otherwise. For the Moon it is advisable to buy a neutral grey filter, because it is also very, very bright.

As for the Messier objects: you can probably view the larger ones. The Ring nebula (M57) will be tough, because it is very small and faint. But open clusters, globular clusters and Andromeda's core will be possible. A few bright nebulas will also be possible, like M42 (Orion Nebula).

There is a good list of all Messier objects over at Astropixels. You should try objects of magnitude 4 or better, and that have a size of more than 5 arcminutes. Otherwise they may be too dim or small to find.