The first time I connected my 27-Inch 4k display to my macbook it offered me to virtually scale the resolution of that display, so I could take full advantage of the 4k resolution while having it in 2560x1440 scaling, so similar to the iMac. After re-connecting the mac to the display it's been virtually scaled to 1080p and everything is just huge. When going to the screen options I'm only offered resolutions up to 1920x1080, even when clicking the options key it'll show up until 4k but it'll no longer scale texts and windows to make it look like 2560x1440.
I basically just want the same options as my macbook screen offers, like I said it worked the first time I connected to the display and I've already tried holding the options key.