Koi keepers often keep sturgeon or sterlets,there are many sub species. But 4ft is a good size in any koi pond. They need plenty of oxygen and do best in cold water,they wont be liking warm pond water with the temps weve had lately.Many koi ponds are built with no ledges etc just deep fibreglassed vertical walls,built to suit koi only ,oh and maybe a sturgeon
,this way the water is kept crystal clear with no vegetation ,infact the only thing in the pond is fish with a bottom drain leading to an external filter is the modern set up
although there are many different set ups.