You can turn off the page-file even with 4gb, I've never ran into a problem, but the "gain" will be minimal at best on a machine that is fast to begin with. I keep one going on a separate drive in all my rigs, and I set it to half the installed memory amount... again, never a problem.
As for 8gb being a noticeable difference over 4gb? Not really, at least not at the present time, unless you happen to be using a memory hungry app/program. Upside to more memory is the availability of faster ram vs. pagefile... larger programs and files load faster because they have more space to use, Games can benefit from it as well, load times for different levels can decrease as "pre-caching" will have more to use... and so on...
The biggest downside to using 4 sticks is you are maxing out the ram slots, which in most cases, puts more stress on the NB. This can result in the inability to OC to the same level you could with only 2 sticks, or even system stability issues at stock settings if the NB is weak... Sometimes you have to give the NB voltage a little bump just to keep things running as they were...
Only way to really know how more ram will work out in your particular setup is to try it out, unfortunately, as results will differ from one setup to another...
Hey Mr. Gates... what ever happened to "Nobody will ever need more than 640k RAM!" huh? LOL