The previous comments list some good targets. When you go looking for them with your 4" telescope, here are some suggestions for giving your students the best viewing experience.
First off, when hunting for deep-sky objects like galaxies and nebula, wait for the Moon to go away. Otherwise it will light up the sky and wash everything out. Also, you're lucky if you live in a rural area, but if you don't, see if you can get away from the city lights.
Bringing a chair or, even better, an adjustable stool will help your students to look through the telescope. This gives your eye some stability and is usually compared to increasing your aperture by an inch or so in terms of the detail you will see.
Have your students sketch what they see. If they've forgotten how to draw, have them take some photos of the Moon through the telescope with their cell phones. It's easy to look at Mars and see a red ball, but if you have to draw it on a piece of paper, you notice all the details that you would have otherwise ignored.
Finally, some color filters might be a good investment. They can help increase the contrast for planetary targets, you can swap them out and compare what you see with each color, and you can have your students come up with an explanation why they see different details through each filter.