916336 - Change the default num_threads to 4.

I was unable to verify that this change actually causes Grinder to use 4 threads by default, due to https://bugzilla.redhat.com/show_bug.cgi?id=916345. I humbly request that the reviewer of this PR would check the grinder log to ensure that it is using 4 threads with this change (and that someone who doesn't develop on F18 review this).