A 4-day trip should cover the main highlights without being too rushed. First day: Arrival and orientation. Maybe they arrive in Hanoi first, then take the overnight train or bus to Sapa. But since the user didn't specify starting point, I'll assume they start in Sapa. So Day 1 could be exploring the town, visiting Cat Cat Village nearby. Day 2: Trekking to more remote villages like Lao Chai and Ta Van. These are popular routes with beautiful scenery and cultural interactions. Including a homestay experience would add value. Day 3: Maybe a visit to Fansipan Mountain, the highest peak in Indochina. They can take the cable car for stunning views. Then in the afternoon, explore another village like Giang Ta Chai or Silver Waterfall. Day 4: Maybe a morning market visit, like Bac Ha Mar