let's say I have a fig tree in a pot like a Hardy Chicago or an RDB (varieties known for producing a crop the same year it had winter dieback) that's 4 feet tall and I bury almost the entire thing in the fall (about 3' 6") and leave it there for the rest of its life.  before temperatures drop too much, I'd trim off the remaining top and cover the area with a layer of mulch for additional insulation.  I am sure people have tried this but I haven't read anything about it yet.  are there any problems associated with this method, and if it works (I can't see any other method being easier than this), aren't I basically guaranteed to get a much bigger crop next year considering I'd have much more growth in the ground compared to trees that were planted normally (while trimming or letting the 4 feet die back)?

Very interesting, welcome to the forum Oats.

Welcome!  I bury as many nodes as possible to be sure some survive the winters.  The plant grows as a bush.  No need to cut off any above the ground unless you want to have cuttings for a back up tree to sell or to share.  You can mulch the above ground parts.  I wouldn't plant it in the fall.  Plant it in the Spring and let it acclimate and have deep roots before winter hits.  Let it sit in a place that stays above 27 but below 50 until you plant it out.  You can plant it when temps will stay above 27 as an estimate.

I'm a little worried about permanently covering 3-4 feet of tree with soil causing significant harm to it since such a thing would probably kill most trees (figs don't appear to be like most trees though, after reading about the fig tree, it almost seems like it's harder to kill it than to keep it alive).  an alternative I have in mind is to bury it about 2 feet instead while keeping the soil separate from the branches with plastic tubes. the same idea would apply.  I'd get to keep more tree alive with very little effort -  cut it down to ground level, throw some mulch on top of it, and call it a day.

like this:
         

A common "old school" method was to prune and bind the tree and, over a period of days, push it over into a predug ditch and burry it for the winter. In the spring it was unburied and pushed back upright. This is the method used for the original Calliope's Red Greek unk, that I have, that has allowed it to survive for many many decades in south eastern Michigan.

You will have a problem,because fig like to have roots at the surface going sideways,at about no more than 8 inches deep and going laterall.
I would guess that ,in the desert,in hot climate the tree will benefit for growing vertical deep roots,(looking for water deep in the ground),but in a climate like mine it grows roots close to surface,because there is a lot of water ,at the surface,3 inches deep or less,from constant rains.
There is a better variation for ,less work:
Plant it normally,at normal 10 inches deep,and later,pile up garden soil 2 foot high,(Cone type mound).
That way the roots are at normal level while the mound of soil protect against Winter,and,excessive rain by forcing water to drain away from tree.

Fig trees are like tomatoes in that they'll grow roots from any part touching the soil.  You should just bury it as deep as you can and forget about the tube.  The tube negates any benefit of putting the trunk underground.  Herman2 is saying that his environment is wet enough that the trees don't have to send roots deep.  Here, in the summer it may not rain for 3 months.  I was gone for 5 years and the tree got no watering.  You can bet that 20 year old tree has some deep roots.

I would like more information so I can attempt to work out any flaws that may exist before I start working on it this spring.  so far it is sounding too easy but I feel like there must be a reason why no one ever talks about doing this.

so far this is what I got:

1) I may have watering issues but the roots will likely figure it out.  I am guessing that if I bury the tree 3-4 feet deep without the tube, roots may start growing near the surface if it feels like that's the best thing to do but would this harm the root system 3-4ft feet down thus damaging the tree, or would all of the roots work together?  I skimmed over a post awhile ago (on a different forum) that implied a weak, higher root system may cause the strong, lower root system to become defunct, however, this was only one post so it could have been false information and the fig tree is apparently far more versatile than most trees, so even if it was true, it may not even apply to the fig tree.

2) using tubes might negate any benefit of burying so much tree but I can't see why (negate is a fairly strong word).  well, I can see the temperature around the tree with the tube dropping at least slightly lower than if I buried the tree without the tube but would it be that big of a difference?  if my insulating layer of mulch on top is good enough then perhaps it'll be fine.  the point of the tube is to remove any flaws that may exist with covering that much tree with soil (by burying it 3-4 feet down into the ground) but if no such major flaws exist for the fig tree, then yes, I'd agree, not using the tube would be a much better option.

3) couldn't the soil 3-4ft down into the ground be way too moist which would then cause rotting?  I don't live in a swamp, get around 30 inches of rain per year (and 30 inches of snow), and I believe the soil is loamy so I don't think it'll be a problem although it may be something to consider.

I'm no expert when it comes to this stuff so any information is appreciated especially since I like doing things right the first time.