Help and Suggestions Needed for 4' x 4' x 2' Printer

[curious aardvark]

Hmm, a 4 foot long belt is going to struggle.
Makes a lot more sense to have a motor each end with much shorter belts.

You're going to need big arse motors anyway.

20watt 12v psu ? Not a chance in hell !

think more along the lines oof 3-500 watt and 24 v.
https://www.amazon.com/LETOUR-96V-24...532295&sr=8-48

Your heated bed - assuming it's heated - will need a mosfet and be directly maind powered anyway. So the psu is just for the motors and electronics.

no matter what you use for a bed, that thing will weight a LOT - it's going to take some really beefed up motors. at least one of these at each side of all your axis would be my minimal starting point: https://www.amazon.com/STEPPERONLINE...9531751&sr=8-6
Assuming the bed moves, from the sketches all I can think is you are building an i3 design with moving bed.
Not how I would go for that size - I'd either make a delta or a fixed bed cube with three axis extruder movement.

The big issue is going to be momentum. The motor not only needs to move the weight of the gantry and printbed - but it has to be able to stop it instantly as well.

board wise think more along the lines of s duet wifi 1.4 clone (or actual duet if you can afford it)
You need something easy to upgrade and add bits to.
The machine is makeable - but you need to drastically uprate your specifications :-)