For a 4x2ft viv you need a 3ft UVB. The UV shouldn't go the full length as you need to allow for a photogradient (light and shade) so that the beardy can self-regulate effectively.
Wattage of the basking bulb can vary and you may need to test with a few different watts. For a 4x4ft I would say a 60-75w would be best to start with.
Dimming thermostat wise, habistat or microclimates are good. I'm not sure if you should plug the UVB into the any of the thermostats but have to admit I don't know the ins and outs of the new digi ones. All else fails get a normal dimming stat and a timer.