I want to replace a blowed fuse 125V/4A but it is rare to find them. Can I use instead a 250V/4A ?
Fuses are designed to protect over-current, not over-voltage.
Therefore as long as the fuse amperage is sized appropriately and as
long as your fuse voltage meets or exceeds your application voltage
the fuse will safely protect.
So, its ok to replace with 250V/4A ?