The technical details are all right (or seem right to me, anyway), but this is too opinionated for my liking. No, you do not need a 4K monitor for software development. Some people might like them, some won't. [edit/clarification: someone rightfully pointed out that nobody will actively dislike a 4K monitor. I was unclear here: I meant "some people won't need them" more than "dislike them"] This sounds like when Jeff…
I love my 3 monitor setup. In 20 years of coding, I finally reached peak productivity in my workspace. No switching between windows, ease of throwing a window aside for monitoring, laying out information and tools side to side. It’s incredible once you readjust your habits and learn to use that new space. I compare it to having a large, wide desk when you’re working on something. Who wouldn’t want that? I was one of…
But I will tell you why multi monitor setups aren't the best for me. It doesn't feel like having a nice big desk to work at; rather, it's like having a separate desk for each monitor, and I have to move from one to the other to use it. With more than one centered monitor, I have to move my head to look between them, or my entire body, so that I can face straight towards whichever monitor I'm currently looking at. I've tried going back to multi monitor setups many times and every time I get tired of it faster due to straining my neck, eyes, elbows and shoulders with all that turning-to.
For me, it's one very nice monitor, with my laptop plugged in in clamshell mode (although now I leave my rMBP cracked so I can use TouchID).
I've also been using a window manager (Moom) with hotkeys to be able to set up three vertical windows on my screen. That seems to be the sweet spot for me: I can have multiple different code editors, or editor+terminal+web, or throw in email/slack/whatever into the mix. (I can also split a vertical column to two windows to achieve a 2 row x 3 column layout, and lots of other layouts, 1x1 vert/hor, 2x2, centered small/large...) I feel like I've arrived where you're at, my perfect setup!
I also still enjoy the 13" rMBP screen, although I can't get to 3 columns, and lately the keyboard hurts my wrists after extended usage. I use a Kinesis Freestyle 2 with the monitor+rMBP which has been absolutely fantastic for typing ergonomics.