To replace the video card in a laptop probably requires you replace the entire system board in it.
You may be able to allocate more memory via the BIOS on boot up (You'll have to press a certain key when the computer shows the first screen probably). I doubt the benefit of this however, because it is all using your system memory. Even though you have "4mb" dedicated system memory for your video, your notebook probably draws extra video memory from your system memory when it needs it. While this is always looked upon as a bad thing by desktop users with dedicated video memory, it shouldn't effect you much since your video chip only runs at system memory speed to start with.
Basically, I don' think increasing dedicated memory will boost performance much, if at all. If you want to try it though, it's definitely worth a shot.