I've been doing my bit on the close queue but also thinking about the larger problem and had a thought.
How many Questions get to the point where they have 4 close votes but then at that point the final result is to leave them open?
If that number is small enough as a proportion of the total close queue then that would suggest to me that the 5th vote is not actually adding a lot of value to the accuracy of the process.
Reducing the number of close votes needed to 4 would make the close process 20% more efficient, which should then cause the numbers in the queue to start trending towards 0 as at the moment the growth in entries in the queue is less than 20%.
The Burn down experiment has certainly made some progress but that progress seems to be slowing and I can't see figures for the total close queue so we don't even know whether we are making any overall progress or just illusionary progress.
Considering the effect of that 5th close vote can actually be directly measured through analysis of the database we can actually make an informed decision as to whether it adds value to the process.