Right firstly I would never recommend 2 dragons in 1 viv especially for a new keeper. First off they would both have to be female or you will have to separate them at some point, even with 2 females chances are you'll have too separate anyway and be sure to do it before harm is done. It can be very difficult to detect the signs early enough before damage is done, be it physical or mental, there's a lot that we as humans won't pick up on. So either get 1 dragon or 2 vivs.
A 4ft viv is fine for life for 1 BD as long as you have a minimum depth and height of 2ft also. This is needed for its size but mainly so that you can create the required temperature gradient. Without a large enough viv either your basking temp or cool end temp will suffer either being detrimental to your BD's welfare.
With you being new to reps and doing most of your research (I assume) primarily at pet shops then they have most probably given you inaccurate advice regarding kit for your viv. Amazingly most pet shops haven't the slightest clue how to properly look after specific reps. I advice you thoroughly research on here on BD. stick around and you'll learn a lot in no time. I will quickly run through what you need.
Heat source. For a 4x2x2 viv you want a 60w light emitting heat source. A standard reflective spot bulb or halogen bulb is what you want, don't buy a reptile specific one, it'll do the same job but be a lot more pricey, this is just to give off heat, you could use a candle in theory if it gave off enough heat. You don't want a ceramic. BD's are attracted to the light and will recognise the light as an area of warmth, it is also easier to create a hotspot with a light emitting source. This heat source must be statted to ensure the safety of your BD, otherwise it could overheat your viv. You want a DIMMING thermostat for a light emitting heat source. If it does indeed get to 10c in the winter at night then it might be a good idea to have a secondary heat source. This would of course need to emit no light at all, so no coloured bulbs, you want a ceramic heat emitter, low wattage as it is only to slightly boost ambient temps, this again needs to be statted, you can use a dimmer or pulse stat for this although pulse is preferable. Set the stat to 15-17c, any higher and it will negatively affect your BD, a temp drop at night is good and in the wild of Aus it often gets to - temps so I wouldn't worry too much. You never want any other heat source for a BD, no heatmat, these can be dangerous as they emit heat from below, not designed for BD's.
UV. You want a tube uv source, it must be a minimum of 10% however the only one I would recommend at current is the Arcadia T5 12% D3+. It is leaps and bounds above the rest at the moment, the tubes last 12 months too as opposed to 6 months so you'll make your money back within the year anyway. You probably won't be able to buy this at your pet shop however it'll be cheaper if you buy your viv and all the kit online anyhow so I'd recommend you do that, try swell reptiles or Surrey pet supplies. If you go for the T5 then you want the 3ft version in a 4ft viv. Mount it in the hot end so it drops off in to shade down the cool end, this is called a photogradient and is again very important to your BD, it is necessary to complete the D3 cycle. Take a look on Arcadias website for more info.
Substrate. This is mainly personal preference after a mass of research however I would steer clear of loose substrates (sand, woodchip). These can cause issues if ingested as they cannot always be properly digested. It is always best to have experience of reps and the rep in question before attempting loose substrates, even with this knowledge it is sometimes still a bad idea, especially woodchip! I'd stick to tiles if I were you, aesthetically pleasing, easy to clean and good at keeping nails trimmed, probably one of if not the best choice for a BD.
Finally you will need a few digital thermometers off ebay (cheapest place). I think I've pretty much covered the basics there and it should be good to get you started. Take a look here
for info on temps and setting up a thermostat, there's a few views there so again its best to research and make your own choice.