[LuckyCat]

I love Cell Saga, so I can't help but get involved. I think the 4th timeline stems from...

Spoiler:

The fact that dead Trunks was in contact with another past timeline, where he went back even further than the canon line, to deactivate the androids ahead of schedule. This must exist because Cell's stolen machine arrived back before Trunks handled Freeza in the canon line. The time jump was set by Trunks, according to the story, so there must be a reason he needed to go back to that specific time.