Hi Veritas,
I think that you will find that there is no 'magical' cancellation of harmonics in the 20 x 4kW ASD scenario vs a single 80kW ASD.
The only way to cancel any of the harmonic currents is to say move half (ie 10)of the 4kW ASDs over a 30 degree phase shifting transformer (ie delta-delta) and essentially make a twelve-pulse drive (at least as far as the power system is concerned).
Otherwise if you truly have a harmonic issue, you could possible solve this by applying either passive or active harmonic filters.
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)