Sorry I can't believe I knocked out that reply without seeing the RISC OS bit, the comments were aimed at running Linux, but some still apply.
Yes RISC OS can use over 1GB, about 3.5GB on my 4GB Pi4B. RISC OS wont take any advantage of USB3, but an SSD is still vastly preferable to a SD card. The increased performance of the Pi 4B over the 3B is very obvious in use. RISC OS can make good use of the gigabit Ethernet, up to
50MB/s to and from my NAS which is a Pi 4B running Linux (using LanManFS and LanMan90, Sunfish NFS is a bit slower). The networking performance also makes VNC sever very usable, so most of the time I remote control it from other machines rather than switching monitors and keyboards.---druck