Hey Wayne,
the Raspberry Pi 4 is for the first time is a major redesign. In comparison to the Raspberry Pi 3 and older, it mainly brings advantages to the desktop computer user, as the USB, Ethernet and all other peripheral chips are now connected more directly to the CPU so are all much faster (Gigabit Ethernet, USB 3, etc.). The Pi 4 is able to run much better graphics standards, i.e. OpenGL-ES 3.0, Vulkan 1.0 and 4K resolution (over two mini-HDMI outputs). Also the CPU is a little more powerful type (ARM Cortex A72-based instead of A53 on older Raspberry Pis). Also you can now attach much more, more fast, and more recent memory (LPDDR4 instead of LPDDR2, and up to 8 GB), which makes it run faster. High amounts of RAM are necessary to run powerful desktop applications. Of course, it also consumes more power. If you want to know what is the main difference in internal redesign between the Pi <=3 and the new Raspberry Pi 4, see here.
It is the first time that a Raspberry Pi is marketed as a desktop computer (while Raspberry Pi 3 and the others are offered as single-board computers, mainly intended for embedded use or as experimental board). You can even get a Pi 4 as complete personal computer, built into a compact keyboard (or as desktop kit to assemble it yourself).
But regarding its use in a small universal 4-axis CNC controller, …
… RPi 3’s seem to have more than enough compute power for our application.
– Doug Coffland