The attack happened “after dark” outside a camp housing some 55,000 people displaced by the conflict and appeared to target the military, said UN spokeswoman Samantha Newport.
A civilian militia source in Rann, which is some 175 kilometres (110 miles) from the Borno State capital of Maiduguri, and a senior military source gave an identical death toll.
They also said eight soldiers were killed in the attack, but there was no immediate official confirmation.
Newport said: “Four aid workers were killed, one aid worker was injured , and one aid worker is missing, feared abducted.”
“Of the aid workers that were killed, two worked for the IOM (International Organization for Migration) in camp management; and one was a medical doctor working as a third-party consultant for UNICEF,” she said.
No details were immediately available for the fourth, but Newport said the injured and missing were both women. All those involved were Nigerian, she added.
Boko Haram fighters killed nine people from the Rann internally displaced persons camp in September last year, as they worked on farms just outside the town.
In January last year, a botched Nigerian airstrike intended to hit jihadist fighters killed at least 112 people as aid workers distributed food.
Commanders at the time called the bombing a mistake and blamed “the fog of war”.
An air force board of inquiry later blamed “lack of appropriate marking of the area” for the bombardment and an unexpected gathering of people at the location.