The 22-person patrol took the four men to Chalong Police Station and confiscated one 10-wheel truck, two six-wheel trucks and a backhoe.
Several cars left the area about 9.15pm as the patrol arrived at the site, between Wat Luangpusupa and a new reservoir to enhance Phuket's water supply, due for completion in 2017.
Work appeared to be proceeding at the site, under the lights of the vehicles, close to the 80-metre mark above sea level. Construction above 80 metres is prohibited.
Held by the raiders were Watcharint La-ongthong, 30, Kanit Sukjai, 27, Somprat Choothong, 49, and Panya Thaiying, 42.
The four were questioned by Chalong police but did not make any admissions and were quickly bailed.
Twelve Navy sailors and 10 Phuket Province volunteers took part in the raid.