Odd. Going by the durability, which I assume is the heat level, 4 adjacent component heat vents only cools by 3 per reactor tick, occasionally dropping to 2. Using 3 drops to a constant 2 etc etc. Or I'm doing something terribly wrong!
Edit: May have been calculating it incorrectly, as I was originally looking at the durability of 60k cells. Using a 10k cell, the heat is reduced by 4 for every adjacent component heat vent.