doc: standardizing on make -j4

Standardizes docs to use -j4 instead of -j8 as it appears to be the
most inclusive recommendation based on discussion in
https://github.com/nodejs/node/pull/9961.

PR-URL: https://github.com/nodejs/node/pull/9961


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>