The largest RTC I have heard of in captivity (in Enlgand) is 5 feet long. I have heard of people seeing them close to 6 feet in the wild and locals reporting (unconfirmed) sightings of them close to 7 feet.
I would say that you would want a minimum of 8 feet front to back on a tank, 4 feet could well end up shorter than the cat.
IMO the smallest tank for an adult RTC would be around 12x8x6high in feet. You have to decide whether you want the fish to survive, or thrive...the more you can provide the better.
A 10,000 gallon would be great, because it does appear that RTC will shoal when kept in a group with enough room.
I would say that you would want a minimum of 8 feet front to back on a tank, 4 feet could well end up shorter than the cat.
IMO the smallest tank for an adult RTC would be around 12x8x6high in feet. You have to decide whether you want the fish to survive, or thrive...the more you can provide the better.
A 10,000 gallon would be great, because it does appear that RTC will shoal when kept in a group with enough room.
true(lol) 
. Atleast they aren't very expensive
people should check aroud before posting.(but we're weired but not carzy people, "It to better see it my dear freinds")