At first, I thought, wow, that's expensive. Then I realized it's for a kit. I like that it has a USB-C port. I hate doing the flip over and over trying to get the right side up.
The greater RAM could be for doing animation or video work on it or running a lot of programs at once which is what you'd be doing with two monitors. With the faster processor in a Pi 4, you can do serious photo editing work where the more RAM you have when doing that, the better the performance.
You may even be able to do video editing work on it with 8 GB of RAM.
This article says you can get by with
only 8 GB if you edit video at only 1080 and close all other programs. But they used Windows 10 and a PC so I don't know if that applies. But video editing does use huge amounts of RAM and a good processor so I bet it does.
You probably don't need it as I expect you're not doing those things.
TechRepublic says that the playing video at 4K caused tearing issues but 1080 worked more smoothly. I don't know if that's an issue of the processor or RAM which has to share RAM with the video chipset and the regular CPU. More RAM, therefore, makes it a better media server.
However, I think it's mostly so you can run more programs and have more tabs open, basically making it into a cheaper desktop replacement. More RAM always improves performance. Windows 10 says you can get by with 2 GB but we all know you really can't. 4 GB is better but when you jump to 8 GB you really see an increase in performance that makes the extra price worth it. If you plan on using it for a few years, the extra RAM will likely be worth it. The price isn't much more I gather.