As mentioned, if you go to settings > general > iphone storage, it'll break down what is taking up the space (into broad media, apps, photos, mail, others) and then give a per-app breakdown.
Besides the apps themselves, stuff that builds up is messages, mail, photos, music, and media. Some of the standard apps take up a lot of space (as an example quick glance, Garag Band (never used – 1.56 GB)). You can offload any apps you don't use, or delete them entirely. In-app settings for your mail and message apps determine how long and how much history they retain. Clear the browser and reading list caches etc. (this makes up a lot of the mysterious 'other' category).
Four GB for a operating system is pretty much par for the course in 2018.
Phones with fewer than 16 GB are a bit of false economy these days, to be honest, especially if you plan to keep them for a good number of years (as I do). It's best to pay for a bit more headroom (contract phones typically shave here, they expect churn to newer models). I have a 64 GB of which I've used 45 (most of which is my music collection). It'll keep me going for another couple of years, I have no compulsion to buy a new phone every two years either. System and apps on mine take up 7.95 GB.