Contrary to popular belief roots do grow outwards rather than downwards but you still may have problems with drainage if this tree doesn't tolerate (and it may) "wet feet" or saturated soil.

Planting holes should be dug to same depth as existing roots of new specimen, so that plant does not settle too deeply after planting. Same material that came out of hole goes back in, without different bulk material (peat, sand, compost etc.) being added, so as not to create a pocket of different texture than soil around hole. This is to prevent problems with water collecting in planting hole during wet conditions or being attracted away from planting hole during dry.

Redwoods can take long periods of inundation. Nonetheless, there is no reason to go down 4', esp to plant a 15-20 gal from HD. No deeper than the root ball. It is unlikely site conditions in the row vary that much from trunk to trunk. I'd dig a hole at proper depth out a few feet from the desired location and see how long that takes to drain. If it is less than a day, for redwoods you are probably good, esp since they need so much water that wet soil is better, esp when they get to be ~20-25 years old or so, as by then the water in CA will be priced properly and people will be irrigating less and trees like redwood will start going away.

Dan

"There are other Aptos Blue trees in a row next to this tree"

In that case, best to get a different clone, so they aren't forced to practice incest. Variation is good!

Resin