I have a blender project with 4 separate scenes which get combined in the compositor. Three of them are quite simple and mostly finished, while I am still working on the main one. However, every time I hit F12 or press the render active scene button it re-renders all 4 scenes, which is a waste of time. Additionally, each scene must be rendered at a different frame, which for some reason I seem to have to re-tell it every time I render, which makes having to render every scene every time an even bigger pain. So is there a way that I can just render one scene?
Not sure if you mean this, but I think you clicked the 'Render' button in the properties panel (which is the same as pressing F12). But if you click the small 'render' button in the compositor (Node Editor) it will only render that scene.
Does the scene you are currently rendering have compositor nodes? If so: rendering that scene requires it to render all the compositor inputs. You can deactivate the compositing step by unchecking the Compositing checkbox in the Scene > Post Processing properties.
You might also consider creating an entirely separate scene for the compositing, then render the other 4 scenes to stills. Use the stills as input to the compositor instead of scenes. This might speed up the workflow as you are tuning the compositing.