By default, my machines boots into RISC OS with a little application to switch to Linux. This will reboot you into Linux after a warning screen. You cannot have RISC OS and Linux running at the same time. You can choose whether the machine stays in the last OS use used or always restarts in Linux.
To work, there is a small widget which needs to be plugged into the USB ports on the back of the machine. This is fully documented in the startup guide.
If you have done this, you will be greeted by the Raspberry Pi OS (formerly known as Rapsbian) screen with some interesting additional icons.
The Files system is also visible on RISC OS and provides a very way to share files. You cannot share programs, but images, text files and documents are all easily transferred. You can also download software as a zip under Linux and then install on RISC OS.
The RISC OS icon allows you to leave Linux and return to RISC OS. The Argon One configuration and unistall icons provide command line tools to control the Fan and power - similar to Fan under RISC OS.
In Linux mode, the FOURtress is a genuine Linux machine with easy integration to RISC OS. I updated the OS to all the latest packages and installed some software. I was a bit disappointed with Chromium (which does not allow you to share Chrome profiles), but Firefox works really nicely. I quickly had the browser synchronized with all my settings and saved bookmarks and 1password installed.
As well as browsers, there is a huge selection of free software readily available and it is easy to switch between RISC OS and Linux. Linux performance is much better than my previous Pi machines and the fast drive helps. There is a performance difference against my 8 core 2019 MacBook Pro (which is also 5 times the price so there should be), but this machine is easily fast enough for general usage and tasks. The only time I really swamped the box was when I opened 25 tabs in Firefox at once. So, unless you are doing some hardcore tasks under Linux, it should be fast enough. This is the first RaspberryPi where I would use Linux for serious tasks rather than just curiousity.
In conclusion, if want a RISC OS machine which is also a competent Linux performer, is nicely integrated and all setup for you, the FOURtress is a strong contender.