Position 4 is quite a peculiar one.
If you got very large hands, you could technically have a 3, a flex on top of the pinkie metacarpal, a 4 and an 5 with no fuss. 
But if you have very small hands, 4 and 5 become a single position, and a pinkie metacarpal would bump both positions 3 and 4. 
Btw, I’m using this guide.
That said, whilst we don’t use a separate convention for Flex placement, I think you are correct: R4 is what best describes this implant. 
But then it’s good to keep in mind that the flex might rotate!
My original placement was perfectly on top of the metacarpal:
And this is the final position:
The implant stayed on the original position until I decided to start using my hand more and more.
As stated on the first post, this decision was to make sure the pocket would form keeping the flex in a position that accommodates my movements (mainly tendons) with the least resistance (thus less chances to cause stress damage to either the tendons or the pocket).
After I begun using my hands to type (which includes a lot of pinkie tendon movement in my case), the implant moved into this new position within a couple of days.
It’s interesting that the base (LED) remained exactly where it was and the implant pivoted in it’s place.
After swelling went down, I could see why this happened:
My tendon adds more pressure into the original position’s tip (near the knuckle), especially when “coming back” (thus adding pressure from pinkie towards ring).
But where the tip lies now, it sits in between my pinkie and ring tendons/knuckles where there is almost no “rubbing” against the tendons.
So if you have something on position 3, then the pinkie metacarpal flex placement might (probably will?) end up overlapping it, unless you force it on a less optimal place, or have quite long metacarpals.
Well spotted!
It was a cheaper version which had the exact same mechanical keys as the Black Widow but shitty internals. Which I modded to my liking and added a 3D printed extension with an integrated switchboard.
Looks great!