Four days is the point at which central Vietnam stops being a coast and starts being a region. The first two days stay by the sea: Son Tra and the Lady Buddha, Hoi An's old town on foot, dinner and a lantern boat on the Hoai River, then a full day at Ba Na Hills for the Golden Bridge and the French Village.
Day three crosses to Hue through the Hai Van Tunnel, with a stop at Lap An Lagoon on the way. The Imperial Citadel and Thien Mu Pagoda before lunch, Hue royal cuisine at the table, Khai Dinh's mausoleum in the afternoon. The last morning is Dong Ba Market or the incense village at Thuy Xuan, then the flight home from Hue or Da Nang.