Yup - practically speaking, 3 hours will be fine. The 4 figure is probably to scare you into not cutting it close, because the formalities of navigating the awful preclearance nightmare at DUB can legitimately take up to 2 hours as a no-status passenger.

(That said, Delta could choose to play hardball if the tour operator is late and 3 hours becomes less, because they did list 4 hours as the minimum. But I think this is highly unlikely.)