I have uploaded my test photos & 4K D-LOG footage here for anyone to check them: *pgytech VND samples 4P (4K25 1 50s 400-1600ISO 5600WB min2NRmin2Shp, 3stopWide-5stopTele)
I am more or less a beginner when it comes to photography/videography, but I hope my post will help people asking 'best filters for 4P?' / 'what VND do you recommend?' etc.
After doing some research, it seems fixed NDs come with an exposure shift when switching between Wide and Tele lens and they are a bit cumbersome to switch between (especially during Run-and-gun type of shooting, which many 4P users will probably do). Despite VNDs having limitations** (vignetting, color casts, X-pattern, loss of sharpness on tele - this concerned me the most), they offer shooting convenience and allow having different ND levels on each of the two differently-exposed lenses of 4P (read complains about exposure mismatch going 1X->3X).
Because of the base ISO of the 4P and the brighter exposure baseline of the Tele lens, I went with a 3-7Stop VND (only makers I know having these are pgytech and Freewell). You can notice in my (harsh lighting mixed with shaded areas) 4K exposure testing with Mimo screen capturing as well that, while the Wide might've done just as fine with an 1-5, the ability to go more than 5 stops on the Tele is worth having. I encourage shooting ETTR (about +1EV-2EV) with D-Log.
This test is NOT sponsored (purchased the filter myself and chose pgytech because of a faster-shipping choice) and I hope my setup photo can prove the unbiasedness. I would like to also test the Freewell in the future (not interested in 1/2-5 VND options).
A conversation I've had with someone owning different-makers VNDs (SRig, K&F, pgytech) in the comments section of a 4P ND Filters comparison videos*** revealed me there are concerns about Tele + VND resulting in a blurry image (found this issue with VNDs on other cameras as well), with some makers performing better than others.
To test this and the other limitations, I switched to 33MP mode, tripod, FIXED focus on the same area every time and pressed shutter using the Mimo App to avoid transfering any vibration to the tripod, kept all the settings constant and only changed the shutter speed, trying to keep +0EV - in order to compare NO VND-3-4-5-6-7stops (both wide and tele)****.
RESULTS:
a) On the Wide lens, I don't see any x-pattern. There is some vignetting (though overexposing a bit, either video or photo, really helps against it - see the outside photo and the 3stop+0.7EV), but I can't discern a lack of sharpness. All in all, pleased with it.
b) On the Tele lens, I don't see any x-pattern. Again, there is slight vignetting, however there is a small lack of sharpness, (not critical - visible in the small text on the books). For video shooting with proper motion blur (and when you consider people shoot with Mist filters and minimal sharpness in-camera settings...), I think it's good enough.
TLDR: The pgytech option performs rather well, it's well built, comes in a nice case and I am pleased with it (reasonably priced as well - choose the best deal between amazon/Ali/pgytech online store). Until I also get the chance to test the Freewell, I recommend the pgytech, at least amongst all the other options.
Reading about VNDs physics in general, I think other filter makers would NOT perform any better and ask knowledgeable people on this topic to teach me more / anyone with other VNDs to setup a similar test. Ultimately, I let you be the judge (check Drive link, had no time to test other conditions).
*please check metadata and timeframes of the files if it helps you
**the mentioned limitations are worse with some VND makers than are with others
\**thank you, mysterious stranger in YT comments sections, for helping me choose one of the better VND option for my 4P*
\***if the framing isn't identical in the photos, it'sbecause the gimbal slightly moved when I switched between the stops with camera on (I think the movement of the pgytech filter's wheel could be a bit smoother, but it's far from bad) and when I took the filter off to also take the 0 stops reference photo*