How many threads does Node actually create?
UPDATE: Since node v6.0.0 you can define how many threads are used by V8 via the --v8-pool-size flag:
--v8-pool-size=num
Set V8's thread pool size which will be used to allocate background jobs.If set to 0 then V8 will choose an appropriate size of the thread pool based on the number of online processors.If the value provided is larger than V8's maximum, then the largest value will be chosen.
4 extra threads are for use by V8. V8 uses these threads to perform various tasks, such as GC-related background tasks and optimizing compiler tasks.