From iaaf site:
611 TECHNICAL MATTERS AND REQUIREMENTS (EXTRACT)
611.5 Scoring – ALL RACES: teams of six runners of whom four finishers shall score. Each race shall be scored separately.
The team results shall be decided by the aggregate of places recorded by the scoring competitors of each team. The team with the lowest aggregate of points will be judged the winner.
In assessing the aggregate, no adjustment to the scoring placings of the finishing teams shall be made in respect of any non-scoring team runners or of individual entries.
Any tie will be decided in favour of the team whose last scoring member finishes nearer the first place.