The step up from Complete Single is not just two more hours. It's full mastering instead of light, and it's every alternate version of the song you'd otherwise buy piecemeal later.
Those versions matter more than people expect. A clean version is required by radio and most editorial playlists. An instrumental opens up sync and licensing. Stems let another engineer or a live rig work with your song. An acapella lets someone remix it. Buying all four after the fact costs $90 — here they're part of the package.
Four hours also changes the session itself. Two hours means executing a plan. Four means you can try a different approach on the hook and still finish.