You'll be able to have more things running without running out of memory (which makes everything slow down dramatically). As long as you have no more than 4GB memory in use, the extra memory makes no difference to performance.
Unless you plan to run memory-heavy programs, I would probably recommend 8GB instead of 16GB.
Most of the 4gb will be taken up by system allocation eg integrated gra[phocs, pagefile etc. An extra 4 will give you best bang for the buck. But it needs to be in a 2 x 4gb kit to be able to guarantee that it'll work OK.