Don't magnolias get big? 4x4x4 would be cruel and unusual punishment for both you and the trees after a few yrs, wouldn't it? Maybe less time because gritty is so conducive to rapid root expansion. I would consider only putting shrubs/patio trees in those dimensions if you could *easily* lift them out and reinstall without damaging the tree or your lower back.

And if you remove them, root prune, then reinstall, will they flop over at the first strong wind? Depends on how much root ball you leave and if you stake.

Also, what if the roots start growing down into the earth under the bricks and expand underneath? Are the bottoms of the planters plain earth? You didn't mention. You'd never get that tree out, but at least the roots would have someplace to go under this scenario. Would this damage your pool equipment? Your planters would be functioning more like raised beds than containers.

I'm not saying what you are proposing is un-doable but I think you have a heckuva lot of potential hurdles to consider.

Boxwood are pretty hardy so I think you can transplant at anytime except maybe if they'll be blasted with 100+ degree afternoon sun.

Re-read Al's epic post about repotting and potting up. You do need to clean out the rootball to prepare for gritty/5-1-1, but you do want to preserve as much of the root mass as possible. I would only prune out the big thick roots, damaged roots or weird stragglers that go off in weird directions.

Oxbow, you are right that most magnolias get big, however sweetbays in this area tend to max out at about 25-30 feet. The ones that I have are about 12 feet now, and they came in a 15 gallon container. My containers are over 400 gallons! So I think that They are good for quite awhile. Plus if I root prune them, I should be able to maintain them at a desired size. Correct? Also, the planters are sitting on a aggregate pool deck. So no worries about the roots going into the ground or my pool. My only concern for root damage is the PVC pipe that runs up the front inside of the planter for the descents. But I had planned to root prune to prevent any issues with large roots.
The trees are currently staked, so I would continue to do so once I repot.
Oh and yes, it will be a lot of work to get these big boys up and out of the planters. Since the planters are 4 feet high and the weight would be significant, I doubt that my 5' 110 pound stature could maneuver them. ;-) Fortunately, I will be relying on my landcaping crew to do the removing. I will just be handling the work that comes after they get it on the tarp. Then I will use their assistance to repot them.
But what I don't know is if root pruning will in fact kill them. Eventually, I will have to do something. And I would like to keep them in the planters. If they lost their leaves each year, like my JM does, I wouldn't even flinch. But since the guy at the nursery has scared me to mess with the roots, I thought I would see what everyone else has to say.
That being said, it looks like you are saying that I can do it, but when? Fall? Spring? I'm not sure how to treat it since it is semi Decidious.
I hope that I am providing enough information to give a clear picture.

Some add'l points:

Root pruning done right doesn't kill plants and in fact rejuvenates them many times. It's something we all have to do if we're no longer up-potting and want to keep a specific plant in a certain size pot indefinitely. More times than not, a well cared for plant can survive and thrive in a pot that looks too small for it. The key of course is the type of soil mix and pretty mindful root care.

It is not a bad thing at all to have bare earth at the bottom of planters. Helps with drainage and root expansion if needed. Plus if the soil mix is fertile enough, worms can get up in there and do their thing.

I'm not sure how hardy exposed magnolia roots are. Seems like the bigger the plant, the bigger the ordeal to extract, prune and reinstall. Hence, more time for exposed roots to dry out and die. You'd need to definitely keep the exposed root ball in the shade this time of year and hose down the roots every few minutes. Of course, if you're transferring into gritty, you'll want to get all the peat pudding soil off.

I don't know how long it would take your tree roots to start circling inside, but you'll want to prune them before this point. It will likely help to fertilize sparingly to keep growth moderate. Also...another thought..tree roots can be incredibly strong...are you worried about them pushing out on the bricks/mortar?

Final thought, don't stake trees unless you absolutely have to (because of wind). And if you do, make sure they are not pegged to the trunk (like a lot of starter stakes you see in the nursery) and be sure to remove stakes within a year of planting. If your pruned mag root balls are still deep and heavy enough, then don't stake.

Perhaps I should switch over to the 511 mix for them instead of the gritty mix, so I could possibly get away with a less than perfectly clean root system, and therebye have the roots exposed for less time. What about placing the root ball in a kiddie pool of water while I work on it, so that the roots remain wet constantly? Is there any reason, that you can think of, that would make this a bad idea?
Also, amazingly I have a lot of worms working away in these planters. I think that they must crawl across the pool deck when it rains and enter through the drainage opening. Or maybe there were a few in the potting soil and they have just multiplied. Either way, I am happy.
I will only be worried about the roots pressing on the bricks if I am not able to prune the roots. I have decided that if all else fails, I will move the trees to a place in my yard. But as long as I am able to get them out of the planters, I might as well prune and repot.
My trees were put in during the fall, so I will be removing the stakes before winter. They are in place mostly to prevent the south eastern winds from shifting them to a crooked position. But by the end of the year they should be able to hold there own.

You want perfectly clean roots with either mix. The only exception is with really fibrous root mats that will get destroyed if you try to clean too vigorously.

The big question is: how often are you prepared to remove, reprune and reinstall? Maybe a better word than prepared is 'willing?' Once a year? 5-1-1 is typically good for 12-24 mos, but you could probably push it to 36 mos (not recommended, but probably doable) if your water schedule is staggered out. Gritty will last much longer, but 400 gal is a heckuva lot of Gritty to mix up. Have you priced it out?

Kiddie pool should work. Fill it and let it sit for a few days to get the chlorine out.

I wish you luck. This sounds like quite the ongoing endeavor. If you pull it off, it will be a great success story. But, knowing human nature, it might be one of those ideas that sounds good on paper but will be a major PITA a year from now in practice.