That is the rating of the insulator. If you use a 120v in a 240v application, the fuse can leak. Just make sure to get one higher rated than your application.
<edit> oh, and a slow-blo is a fuse that is rated @ 4a, but can take 5-6 for a second or 2 and not blow. Fuses are basicly made to prevent direct unfused grounds . Those start fires and damage circutry. if a device pulls 4a, it can usually handle a certan amount of overage for a few seconds without any damage. ( depends on the unit / quality ) In that case, a slow-blo fuse is used.