I totally agree that those dimension would hold a RTC very comfortably for several years. Just know it may not be the last pond you build. It will take several years to reach 4 feet, but if give a healthy environment, the right food, and good water quality, there is no reason it wouldn't.
As for a 4' wide tank being wide enough for a 4' long catfish I beg to differ. When was the last time you paralleled parked? These guys aren't super flexible when they're big like that, and they need at least a little bit of margin. A full grown adult would not be miserable with 4' wide, but not "comfortable".
As for being inactive... My cats are incredibly active. Now, granted, I have a RTCxTSN not a RTC, but I have quite a variety and every large cat I have is incredibly active. If I go out and anyone is sitting on the bottom, I'm immediately checking for problems. The only time these guys are quiet is the rare occasion when I've fed them too much, or when they were cramped in the 180 gallon while I finished the grow out 600. When they were all cramped, they barely moved around. Now they're everywhere, following me around, playing in the current, punking each other, etc.
Here is a link to some giant cats that are still growing like crazy here on MFK:
http://www.monsterfishkeepers.com/forums/showthread.php?t=285048&highlight=pond+catfish
So, in all you'd do will with either of those sizes. The 10'x6' might actually be better since he would have a nice wide easy turn, while still having a diagonal of 11.7 feet. The 12'x4' would give him a little longer diagonal of 12.6', but he wouldn't turn as easily when he gets large. Again, either way, you'd be good for probably 3-4 years.