I've been trying to model a set that provides continuity between adjacent strings to make my sound more even across the strings. Trying to limit big changes of string mass over the poles of the PU.
Take the sound of open G: play the same note on the other strings and how do they compare for similarity?
The best 6 string set I could find was 130-100-75-55-40-30.
30:######
+10
40:########
+15
55:###########
+20
75:###############
+25
100####################
+30
130##########################
The sizes increased by an increasing amount. It is not a linear change but exponential. A curve, depicted above.
Of course it had to be playable as well. This selection was limited to 005 graduations of string sizes, but worked OK for me. I got used to slightly lighter strings but my sound was more consistent over the range of the bass, giving me an EQ advantage from the very start.
I took the same progression and rescaled it for 4 string. The result was 105-80-60-45, a slightly heavier set than the 6 string.
A linear set will also play well eg 105-85-65-45, but the EQ advantage is not as good IMO.
This heavier range is possibly why 4 string basses seem gruntier, but without the range of a 5/6 stringer. It needs a compromise to increase that range without changing the scale of the bass. Fanned frets might help a little.
If other odd sized strings are available then the nos could be adjusted to lessen the compromise, and improve playability. I often detuned to D (thanks Mr Hipshot) but a heavier string might help. Life is such a compromise with perfection.