In a scene reminiscent of the Cold War, Swedish naval ships, helicopters and ground troops combed the Stockholm archipelago for a fourth day Monday for signs of a foreign submarine or smaller underwater craft that officials suspect entered Swedish waters illegally. While Sweden hasn’t linked any country to the suspected intrusion — and Moscow suggested it was a Dutch sub — the incident sent a chill through the Baltic Sea region, where Russian forces have been accused of a series of border violations on land, sea and air in recent months.