1. No matter what operating system you're on, you'll eventually run into an application that doesn't render in high dpi mode. Depending on the OS that can mean it renders tiny, or that the whole things is super ugly and pixelated (WAY worse than on a native 1080p display)
2. If the 4k screen is on your laptop, good luck ever having a decent experience plugging in a 1080p monitor. Also good luck having anyone's random spare monitor be 4k.
3. Configuring my preferred linux environment to work with 4k is either impossible or just super time consuming. I use i3 and it adds way more productivity to my workflow than "My fonts are almost imperceptively sharper" ever could
My setup is 2x24" 1920x1200 monitors - so I get slightly more vertical pixels than true 1080p, but in the form of screen real estate rather than improved density. I also have 20/20 vision as of the last time I was tested.
My argument in favor of 1080p is that I find text to just be... completely readable. At various sizes, in various fonts, whatever syntax highlighting colors you want to use. Can you see the pixels in the font on my 24" 1080p monitor if you put your face 3" from the screen? Absolutely. Do I notice them day to day? Absolutely not.
I genuinely think 4k provides no real benefit to me as a developer unless the screen is 27" or higher, because increased pixel density just isn't required. If more pixels meant slightly higher density but also came with more usable screen real estate, that'd be what made the difference for me.