With a 4ft tank you should definately be thinking about a full capacity external cannister or two 3/4 capacity external cannister filters. I'd suggest at least 850L/Hr flow rate, which would give you 5x turnover. Note that anywhere from 5x to 10x turnover should be of interest, 10x being a rate of movement that would help keep algae away, but which can also be achieved with the help of extra powerheads.
I'd suggest an eheim pro2 model 2028 with a hydor inline heater as a pretty nice mid-range filter/heater system for this tank if not going with dual filters. Besides Eheim, I 'd also look at Rena, Tetratec and Fluval filters.
Major categories of filters overlap each other. Going from small to large in the most important parameter, media volume, you generally find these categories: internal filters, Hang-On-Back (HOB) filter, external cannister filters and finally sump filters. For your size tank, a pair of large HOBs might be minimal (but Aquaclear makes the most flexible of these) but might work out to less money. Next up would be a good large external cannister, like the eheim I've mentioned, next more expensive would be two slightly smaller external cannisters (by the way, the advantage of two is that you can alternalte the cleanings and each serves as a backup to the other in the event of failure and the two outlet spraybars might give increased flexibility in directing the water around the tank) and then finally, building a sump system (essentially another smaller aquarium filled with filter media in the cabinet below the main tank) would be the most involved and would probably be overkill for your tank, but its all individual choice of course.
Good luck with your research. Don't think of setting up a tank and getting fish in it as some kind of desparate race, instead think of the research and learning and setup and one of the most rewarding and interesting phases of the hobby. If possible relax and enjoy it! That's what this hobby is all about ideally.
~~waterdrop~~
