Upgrade to a 64-bits version of XP. :-) Actually, most 32-bits applications will work on a 64-bits system. Am not sure if your laptop can handle a 64-bits OS, but if it can then it should be able to use more of your memory.
About the /3GB setting that you can add to your boot.ini, it's useless unless your applications support this option too. They need to be compiled with the /LARGEADDRESSAWARE flag set.
Then there's the Physical Address Extension, which is an Intel technique to extend the memory from 32 to 36 bits, thus you could have up to 64 GB of RAM. But again, applicaations need to be developed to support this, else it's of no use.
Btw, Windows does use the 4 GB that you have. It just doesn't provide this memory to your applications. Applications can normally use up to 2 GB of RAM. Windows won't be able to use more than 4 GB, unless you use a 64-bits Windows version or use PAE to extend memory to 64 GB.