Looking fwd to your new tank build then. Good dimension on choosing 4ft tank, as for depth its up to individual preference. I would recommend 1.2-1.5ft at least. Width I think 2ft is a gd choice, easier for rock placement & scaping & give a more 3D effect to your scape. As for cooling method, compressor will be a better choice in term of efficiency & save u some $ in the long run.
My 2 cents.