The attack comes as Kurdish forces from the People’s Protection Units (YPG) spearhead the fight for the Islamic State group’s Syrian stronghold Raqa as part of a US-backed alliance.
The Britain-based monitor said the blast hit a checkpoint manned by the Asayesh security forces in Hasakeh province, around 20 kilometres (12 miles) from the town of Ras al-Ain.
At least two of the dead were Asayesh members, the monitor said, while the identities of the other two were not yet confirmed.
Syrian state television also reported the blast on Tuesday and said four people were killed.
Kurdish-controlled areas have come under regular bomb attack, with IS often claiming responsibility. There was no immediate claim for Tuesday’s incident.
The US-backed Syrian Democratic Forces, which is dominated by Kurdish forces, is battling IS for control of the jihadist group’s Syrian stronghold Raqa.
The militia now controls around 35 percent of the city, according to the Observatory.
More than 330,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.