Originally BICC made NACA4 exclusively for Naim, but Linn persuaded BICC to supply them with an identical cable (other than the colour), which was sold by Linn as K20 (and sill is today, I believe).
Naim were not happy with ‘their’ cable being copied by Linn, so Naim found another cable manufacturer to produce NACA5.
I tried NACA4 and K20 in my vintage Linn/Naim systems a while back - always with Naim boltdown/CB amps. They sounded surprisingly different considering the construction/history. NACA4 was cleaner and clearer with perhaps a touch more swing to the music. K20 was a bit muffled by comparison but the music made more sense to my mind, was more coherent.
They are both cheap enough, so perhaps buy both second hand and sell one set, although A4 is not so easy to find these days. Or if you’re happy with either from a musicality perspective, then just pick the one that better suits the balance of your system - i.e., use the cable to tune the sound more to your liking.