Okay I'm going to recommend a tension for advanced techniques / Ebow / tapping / chording / bending. Assuming you want to tune to EADG one octave up. I think the set I mentioned above will be good:
G .013p 17.3 lbf
D .018p 18.6 lbf
A .027 22.2 lbf or a .028 is fine too
E .040 26.2 lbf
lbf = pound-force of tension
I designed this set using the
Circle K Strings tension chart pdf
For comparison, a 40-100 bass set has roughly 40 lbf, a 9-42 guitar set has 15 lbf.
I have just bought a fretless 4 which I have strung as a piccolo in fifths DAEB .045 .028 .016p .010p using D'Addario XL nickel roundwounds. I find D'Addario XL nickels and prosteels work well at low tension, possibly due to being so flexible.
I'm using Rotosound plain steel guitar strings for my highest 2 strings, they're just long enough. I detach a bass ball-end from a dead string and thread it onto the guitar string, and slide it up against the guitar ball-end.
Alternatively you could buy 4 single strings from
Circle K Strings I think they will be ideal due to being ultra-flexible.