Looks like you have a good start. Did you mean 8x4x2 feet working area? I have a 5-axis build with the same envelope planned (though not started, just collecting parts and finalizing the design at this point).
What is your intended work material? Given the design, I am guessing wood and foam.
Can you tell us more about your BC head? Are those planetary gears? What sort of backlash/torque/ratio rating do they have? What is your spindle power rating?
What is your Z column constructed of? The same steel tube? How do you plan to keep it from falling on motor shut-down? Have you considered the vibrations you will see when the Z column is fully extended?
I assume you are using rack and pinion on the long axes - what about Y and Z?
Do you have
CAM software chosen?