I see a lot of advanced users making advanced recommendations in this thread.
In order to get this familiar with Qubes, we all probably had computers with a lot more than 4GB of RAM to help us.
Additionally, I would say that people who are this familiar with OS details (using another firewall OS, limiting dom0 memory, maybe using commandline clients for certain tasks, …) fall into a certain category of nerds / hobbyists / IT enthusiasts, which typically have rather advanced computer setups and will rarely have to actually deal with a 4GB RAM limit.
On the other hand, I would say that people using a computer with 4GB of RAM would typically not be considered “power users”. Because, let’s face it, 4GB is low even for a regular OS. Additionally, an installation of Qubes on a computer with low memory capacities will most of the time be your first installation. Maybe you had an old computer lying around and wanted to try out this shiny new thing you discovered.
Turns out, there is not much trying out if every other time you want to start a VM it times out due to memory limitiations. I would argue that for someone without prior knowledge to Qubes, it is close to impossible to use it on such a device.
Yes, technically it might be possible, but I think the minimum recommendations should reflect what you can actually expect from such a setup. At least, the docs should probably say something like “at least 4 GB RAM (if you are willing to make compromises and know what you are doing), 8 GB will work better for most starters”.