Well, that would depend on how deep/tall the drainage layer is, and that info wasn't mentioned. The point isn't specific or unique to 4 ft tall pots, so the point isn't moot. While a deeper pot has a higher % of soil available for root colonization unhindered by a layer of saturated soil than one shallower, the limiting effects of a layer of saturated soil are still in play for either pot.
The comment was provided as cautionary information so growers wouldn't think a drainage layer of pebbles or gravel does anything to promote better drainage in ANY pot, because in fact it makes problems associated with water-retentive media worse, no matter how deep the pot.
Al
