Alnico IV definitely does exist and was certainly used throughout the 50s and notably is found in original PAF humbuckers(along with AII and AIII).
Alnico IV - AL 7% Ni 14% Co 24% Cu 3% Ti 0%
Br: 8000 Gs, Hcj 660 OE, BHmax 2.0 MGOe
Alnico IV is Isotropic and is made specially for us based on original samples from the 50s.
Alnico IV is definitely not sub standard or low charged Alnico V-Alnico V, VDG and 5-7 is also heat treated differently.
All reputable pickup makers such as Lindy Fralin and Tom Holmes acknowledge the existence of Alnico IV and use it in their pickups just as we do too.
Most importantly the tone produced from Alnico IV powered pickups is very different to AII, AIII and AV.After all that's what we're all interested in-tone!