Do all apps have a demo/trial version? Someone suggested a pay-for app in another question and when I looked it up, it seemed it didn't have any trial version that has the feature for which I'd pay.

Now, I will not pay for something I have never seen working, even if it's only 4€ and so I was wondering whether I missed something or whether it's usual for pay-for apps to charge before you even can try it?

(This app, Moon+ Reader, does have a free version, but I'm specifically interested in a feature of the pay-for version, so the limited free version doesn't help.)

It seems like a very long time (maybe never) that I payed for any software on my desktop where there wasn't a demo of some sort to try out the key features you payed for.
You get 15 minutes trial of all paid apps listed in Play Store. Its Google's policy to refund money if you aren't satisfied but you'll have to claim refund within 15 minutes. After purchasing an app, just look for Refund button on app page in Play Store app.