OK, Season 3 has no Martha in
302, 304, 309, 310, 312-3, 314 (mentioned but doesn't appear), 319, 320
Season 4
405, 407-9, 412, 414, 416, 417 (mentioned in dream sequence), 418-20
Which makes 38 out of 88 episodes without Martha, so she's in about 56% of episodes. If I say "more than half" it's close enough.
