The short answer here is no 2GB is the limit

The long answer:

There are a few different things interacting here. First you have the memory modules address lines connecting to the sockets contacts. In this case it appears the sockets address lines are active which is why the 2GB module is working. But, that is likely not the real issue holding you back. Its the address lines used by other devices within your system the biggest is the graphics services. In older systems like yours these addresses used by the GPU's VRAM can't be moved. Which is why you can't put more than 2GB of RAM in the system.