Reduced the number of threads used for testing from 8 to 4

Given the present highly serialized nature of the available modules, running 8 threads is a complete waste of resource. Dropping to 4 should still give us more than we need but hopefully free more resources for the IB RelVal tests.
Merge pull request #2537 from Dr15Jones/reduceRelValThreadsTo4

Reduced the number of threads used for testing from 8 to 4