The MFT will fragment only if it is unable to expand contiguously. This can occur if the free space on the volume drops to less than ~ 85%, and/or if there is a large number of small files on the volume (Very small files ~1kB are stored in the MFT itself).
The capability for defragging the MFT is built into XP and higher versions, but in XP atleast, the default defragger actually does not utilize this feature, instead a third party utility such as Diskeeper is required. Most of the MFT can be defragged by Diskeeper without requiring a boot-time defrag (unlike previously), and a boot-time defrag will fix the remainder.
Anyway, with only 4% of the MFT in use, there is absolutely nothing to worry about at the moment :)