The version you are seeing released with XP has a SATA 160GB HD and as such, is slightly thicker to accommodate it.
There is no upgrade pack, primarily because XP doesn't work well in standard config on an SSD, so you will have to get your own version of XP and adapt it to work functionally.
With only a 4GB SSD, the key to installing XP successfully is to keep the installation size to a minimum, and then deal with the issues XP has on an SSD.
Here are some links to guide you.
i64X.com - nLite Windows XP Install on Asus Eee PC
Making XP pro SSD friendly - OCZ Forum
Solution : Core SSD's freezing and stuttering(WinXP) - OCZ Forum