It is only good for machines that have 1GB of RAM or lower. You need to have a USB Thumb Drive that is DOUBLE the amount of RAM you have. That is to at least be effective. It gives you minimal speed gain. We are talking nano-seconds. Something not noticable by the human eye.
That is not 16GB of RAM. At best it would be like adding maybe 1GB. USB is SLOWER than RAM by a long shot. The purpose of ReadyBoost is for older PC's trying to run newer technology.
Simply put it is for dinosaurs running Vista.
Vista stores some information on there that is very rarely used or accessed. So that it can free up the RAM for use by things that are constantly being used and accessed.
So no with 4GB of RAM you will NOT see a boost. The USB Speed will be slower and it might actually force your system to work harder.
If you are runnign DDR2 RAM of at least 533MHz you are already moving faster than most transfer speeds for USB. Not to mention this eats up USB Drives like candy since you need one that has a ULTRA high write aspect. Cause you can get a million writes to the USB in just a very short period of time.