I'm a professional user interface designer/coder and do personal digital art in my spare time. When I first started my job in 2013 I was given a Lenovo T430 with i5/4GB and Adobe CS6 Premium. I also put in Visual Studio for web development. I rarely had an issue with 4GB RAM. When I did it was under the condition of running Photoshop, Firefox, Chrome, IE, Outlook, Visual Studio, and probably a bunch of corporate background stuff together. In that stack of things, it was the three browsers (especially Firefox) that ate the most memory. And I can't even remember what these "issue" times really even involved. Maybe a bit of swap delay.
I can say that when I absolutely have hit a 4GB ceiling was during development running Visual Studio, full IIS, a SQL Server instance, browsers, and a few bits of productivity software at once. That was 6 years ago on a large development project. You can also do it if you're running one or more virtual machines. So you can absolutely max out 4GB, but it's not under what I would consider normal usage situations.
I went with the i5/4GB for my SP4 choice to get the power but never seriously considered upgrading to 8GB. Chances are in 2 years there'll be a SP5 or SP6 which I'll be interested in so by my thinking I really need to be stopped dead in my work without 8GB to justify the cost. That just doesn't happen for me.
However, the SP4 is also my secondary machine. My desktop has 8GB and dedicated graphics so if I need to do a really big job I have a fall back. YMMV if you are using the SP4 as your only machine. Honestly, going back to my desktop happens more for the ergonomics of having a much bigger screen than power/RAM.