Sadly, most 4e adventurers would probably think “There are several of them, so they must be weak.”
Sometimes you want to step out of the 4e assumption that every fight is level-appropriate. Maybe an area is particularly dangerous. If you just spring a killer fight on unsuspecting players, though, you’re just being arbitrary.
Here’s one way to take a middle course: when the players wade into combat with the first light ball, they find that it is a high-damage solo monster. During the battle, the next-closest light ball might drift ominously closer, but doesn’t join the fight.
When the players stand, bloody but victorious, over the creature’s dying sparks, they look down the hall and see this:
Possibly now the PCs will start thinking about finding an alternate route.