How to scale down 4k monitor

One thing you can try is to hold Alt while selecting Scaled in the Display pane of the Displays view in System Preferences. On my Mac, that increases the selection of resolutions (and the effect of that is sticky; if I hold Alt and click again after reverting to Default for display, the list shrinks back to what it was initially, but clicking back and forth without Alt shows me the same list of resolutions each time). It's possible that won't help, if your monitor simply doesn't offer up intermediate resolutions, but that might not matter if it's the GPU that does the scaling rather than the monitor.

I tend to prefer to choose a mode which uses the native resolution of the display, because it maximizes the crispness. It's possible your 1080P is actually 2160P, but described by System Preferences as Looks like 1920 x 1080; that uses four times as many pixels per character in the font selection, compared to a true 1080P. Depending on whether your monitor is treated as equivalent to a Retina display when connected to your Mac, what it's calling 1080P could be either (a true scaled 1080P, or just a better choice of fonts). If the applications you commonly use are sufficiently capable of changing font size, you can increase the font size (if you're using 2160P) or decrease the font size (if your 1080P is like a Retina 1080P). Then again, if the pixels are small enough, an intermediate scaled resolution (if System Preferences offers that with help from the Alt key) might look OK (for me the pixels on a 28" UHD display are big enough that they don't look good to me scaled, but then again the display on my 27" Retina iMac has completely spoiled me).

One problem with relying on applications to scale fonts is that it doesn't change the menu bar and dock, but that does allow you to adjust the content you probably spend the most time looking at.

In any case, it's probably worth your while to try the Alt trick to see if that changes the list of resolutions.