In this universe, Society has structured itself so that, while they still have modern tech, Adventurers needn't look far to find a wild forest or cave of monsters. How would a village look to be self sustaining and only about four acres, not including the farmland needed to sustain it?
Can't be done with modern tech. To be completely self-sustaining you would need a power plant, manufacturing capability for machine parts (even current CNC and 3D printers can't replace all the machine shops), some way to produce textiles for clothing, and of course electronics/batteries, which is a HUGE undertaking that requires very specialized manufacturing systems with some rather toxic by-products you don't want right next to your living quarters.
Even medieval villages were not totally self-sufficient. Iron ore, pottery, clergy, etc were all things that usually came from elsewhere. A village might have a source of something nearby (a mine, for example) that was it's lifeblood for trade while producing at least some local food, wool, lumber, etc but trade was always a thing for agrarian societies.
You COULD have a self-sustaining temporary work site, something that had a big supply of spare parts, a generator, fuel depot, and perhaps a renewable energy source like wind or solar. Then all you would need is a food source and you could operate for a relatively short period of time independently. So a pop-up town around the entrance to a new found dungeon in a remote region, for example. But modern tech is far too interdependent on constant resupply to be self-sufficient. Maybe in a few generations we'll have 3D printing/nanite construction that can literally process raw matter into refined consumer items, but we are far from it now.
It would be a giant office/housing tower building, that's your only option really if you want it to be that small. Vertical farming could be employed, with floors of the building being used entirely for growing crops / raising livestock. Underground floors could be used for manufacturing, etc.
The more things you require them to be self sufficient in, the taller the building would have to be, so there may be some limitations (I'm not sure how tall we can comfortably build towers), but I see no reason why something like this wouldn't be feasible with modern technology.