I've read somewhere in the 'net that some hams use this core for building baluns and ununs, and I decidde to satisfy my friend's request using this core.
I'm inspired on the work of another italian ham radio operator, IK1ICD (you may look at his job HERE )
I arrange quickly a 4:1 un-un using the usual schematic:
I use 2x1 mm speaker wire, 2 and a half turn + 2 and a half turn + 2 and a half turn (...whew! simpler to look the picture below than describe it!)