Growth rates are determined for 1-gallon sized plants. At this point and after a year of being in the ground, they should grab footing and on their second or third year, the plant(s) will begin growing at the rate "prescribed" for that particular cultivar.

If four feet, find the growth rate of those cultivars and do the math. Again, they won't begin growing at the prescribed rate until the roots have grabbed soil.

Dax