Lethargic, sluggish, torpid, comatose are comparable when they mean being by constitution or condition physically and often mentally inert or inactive.
Lethargic usually implies either a constitutional or a temporary or pathological state of sleepiness or drowsiness that makes for slowness in reactions, responses, or movements, or for temperamental apathy.
Sluggish applies not only to persons but to whatever by its nature moves, acts, or functions; the term implies conditions which create stagnation, inertia, indolence, or inability to proceed at a normal or usual pace.
Torpid suggests the loss of power of feeling and of exertion; basically it implies the numb or benumbed state of a hibernating animal, but in its more common extended sense it implies a lack of the energy, vigor, and responsiveness that one associates with healthy, vital, active beings.
Comatose basically implies a being in the state of profound insensibility called coma that results from a disease (as diabetes or uremia) which spreads poisons through the system or from severe injury. In extended use comatose implies the stultification of extreme lethargy.