I wouldn't buy any size less than 6x8 since I prefer to use the mapping in pen mode which sets the tablet area to function like the screen area. 4x5 would probably only be good for mouse mode if you like lifting and dragging the pen around like a mouse and if you like taking notes. A small size is not good for an artist…
I am aware of monitor switching in the Wacom control panels, but it' not functional enough for me dual monitors. I would prefer to be able to use pen mode with both monitors and be able to switch between which monitor is for the tablet with a push of a button. But for now maybe I`ll just rotate my 2 wide screen monitors to…