4166 is a little closer to H4895 than VarGet. It seems to be bulky and this limits its use in small case cartridges. I'm running a series of range tests now on H4895 / VarGet alternatives using a 223 Rem F-Class rifle and the 77gn SMK as the test vehicle. I was surprised by the absence of 4166 loads given that burn-rate wise it's nominally close to H4895. When I say absence, I mean 100% absence bar Hodgdon's own 'Annual Manual' in the 223 and similar cartridges. This raised concerns in my mind that there was 'something funny' about this powder, say a tendency to unpredictably pressure-out or suchlike, but it appears more mundane. It looks like you can't get enough powder into a 223 case to give the MVs people expect. My first go with the powder was cautious given the above concerns and topped out at 23.0gn in a chamber whose freebore is around midway between Wylde and PT&G 'ISSF' and 2.4"COAL. MV in a 30-inch barrel was 2,800 fps, same as 23gn H4895 more or less. My top load with the benchmark H4895 was 24.4gn for 2,975 and I'm looking for similar performance from potential alternatives.
My second lot, loaded but yet to be shot, goes up to 24.4gn, but charge compression started at around 23.6gn in current 30.5gn water capacity Lapua brass despite the longer COAL. I now have QuickLOAD v.3.9 that includes the powder and it suggests it pressures out rather earlier than H4895 and similar powders. In a charge table with 105% fill-ratio and 55,000 psi PMax limits in my set-up, 4166 is second lowest MV out of 24 suitable UK available powders. QL is of course just a model, so I'll have to see actuals when I next get to the range. This doesn't promise well though for those limited to 2.26" COALs either through freebore or magazine-use. As that's what the bullet and powder companies all work with, and use of the SAAMI 223 Rem chamber with its 0.025" freebore / sharper leade angle constraints, one can understand why nobody other than Hodgdon bother to list the powder, especially given so many alternatives.
There seem to be fewer 4166 constraints in larger capacity cartridges, although even there I notice those manuals which include in 308 Win and 30-06 appear drop it from heavier bullets where loads continue to be provided for H4895/VarGet.
On Lovex SO62 (SW 'Precision Rifle'), I had thought it slightly faster burning than VarGet until I tried it in the above 223 / 77 set-up. It was around 100 fps down on VarGet on the first go with near same charges and needed an extra 0.6gn at the top end to match VarGet MVs, a fair increase in this cartridge.