The SS4000-E was manufactured with 256 MB DDR SDRAM memory. The memory in the SS4000-E is non-upgradable.
The firmware on the SS4000-E doesn't support additional memory and there is no benefit to increasing available memory. The using a larger memory size shouldn't cause a failure, the system would just not use or even be able to address part of the memory.
Memory larger than the 256 MB DDR SDRAM memory has not been validated by Intel.
It could just be a function of the older architecture causing the perceived low speeds.