CLs will never physically outgrow a tank like say a silver arowana would, but the 4' length will slow their growth long before the look "too big" for the tank and will ultimately shorten their lifespan. I've watched a bunch of healthy clowns in a 4' tank slow down and stop growing at 5-6" and none made it past ~10 years old, which when you consider they could love for 20+ years and get to ~12" is pretty poor IMO
4' length will do for a while, but IMO if you want to do the right thing and keep them happy, healthy and growing then get them into a 6'+ tank with plenty of current by the time they hit 4".