Upgrade the oxygen and water fileters, the generator, the workshop, and both doors (internal and external). The workshop must be upgraded to level four. All components of both filters and the generator must be upgraded to level four. The doors must both be upgraded to their maximum level. Although other items can be improved (shower, bed, toilet, pantry, water butt, stove, etc.), they will count as discrete items and not upgrades. You do not have to deconstruct your Workshop 1 items and replace them with Workshop four items.