The more the memory the longer it takes for the PC to sleep or wake, but once the notebook is 'on' the differences from a 2GB through an 8GB module is very (very) small.
As far as a 4GB vs 8GB module -- the spec's are 4GB modules not 8GB. If an 8GB module 'works' then 2x8GB would also work. The differences between Single Channel (1x2GB, 1x4GB or 1x2GB + 1x4GB, etc...unequal) is very small in comparison to Dual Channel (2x2GB, 2x4GB, 2x8GB or 2x16GB...equal). Tough I recommend Dual Channel as your best choice.
Most importantly, unless your 'Free' memory is zero then there's no need to 'upgrade' your RAM -- you'll never notice the difference. Now 'if' your 'Free' memory is listed as zero then you might want to consider a memory upgrade. Most folks unless they're Rendering, Large Databases, or Game typically have no need for more than e.g. 4GB (2x2GB).
To speed up your notebook
by far the best purchase or upgrade is an SSD, plus an SSD uses half or less power.
(note) Free memory: