If they start feeding ok in your 45 gal, they will outgrow it in ~ a month, I'd say. Four robust-bodied 6" channels in a 45 is already a bit more than I'd settle for - too little water, too much fish mass (equals high instant ammonia and nitrite - not enough water to dilute them fast)... not even to speak of the swimming space.
Starting from under 6", channels will grow to 1.5' in the first year, 2'-2.5' in 2-3 years, 2.5'+ in 5+ years under average proper care.
They can squabble a bit when bigger. More so in a small space, less so in a large space.