The problem is what you can do without harming the trees. Maybe try scraping the sod off with sod cutter or killing with glyphosate, although that would take longer. Adding depth of new soil sufficient to plant hosta from typical one (or two) gallon pot size might harm tree roots, by reducing air supply (suddenly feeder roots are buried deeper, no longer near the surface where air is). Probably have to get by with a minimal depth of well aerated soil, starting with smaller hosta plants and/or fanning roots out so they will fit in shallow layer of soil.

Trees vary in tolerance of fills, good general policy is to fill as little as possible and use highly aerated fill. Definitely do not run a rotary tiller through roots of trees you want to keep healthy!

The one thing that stuck out to me was your "4 feet wide" phrase. I am picturing a long narrow snake-like bed wandering from tree to tree. That does not sound too appealing to me, but it is hard to say without a picture. BTW, maple trees have a reputation for being very hard to garden under because of their aggressive and thirsty surface roots.

Occasionally nice pictures of such gardens show up on the Hosta forum. I have not stopped in there lately to know if any have been posted recently.

- Brent