For owners of Emisar D4V2 wondering about the difference between D4V2 and S21D, I have a more concrete answer with arrival of the Oppel Lightmaster.
First, wrt quality of beam, from picture below I see beautiful beams from D4V2 with quad 219b LED and 10622 clear TIR. When you are near perfection like D4V2, there is nothing the S21D could improve upon IMHO.
Where the S21D is better, is prolonged brightness. In real life use, I have noticed that the D4V2 starts bright but then becomes too dim to use for me as a walk light. Photographs also show a dimmer, smaller beam versus S21D. With the Lightmaster Pro, I now could roughly quantify the stepdowns of D4V2 vs S21D 219b 4500K using ceiling bounce method. The number is percentage of lumens of max Turbo start. The 2 lights start at roughly same brightness level, but watch the difference in subsequent stepdowns.
D4V2: 100% start, 40% at 2 minutes, 28% at 5 minutes, 22% at 10 minutes
S21D : 100% start, 85% at 2 minutes, 68% at 5 minutes, 39 % at 10 minutes
Roughly, 90 seconds after Turbo start, Convoy’s lumen values are twice those of D4V2’s.
BTW, prolonged brightness seems to be the hallmark of all Convoy flashlights I have, big or small. They do all run hotter than other lights (60° C at the head is par for the course), and they do seem to stay bright. Conversely, very rapid/significant stepdowns seems to be the behavior of my small Nichia lights with Anduril, Wurkkos TS21(even worse) and D4V2. PS Still love all my Emisar’s, for many reasons 
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