The idea that shorter needles are better are based on a few simple concepts. a) Better patient compliance - shorter needles are less scary and may hurt less, so people with take their shots when they are supposed to. b) Less risk of IM injections (which are a perfectly reasonable thing to do for speed).
But the data they they used to say 4mm is equivalent to 8mm was cherry-picked to get that conclusion. If you look at the referenced studies, there are significant problems with them, and some of them even contradict the conclusion.
The first cross-reference:
“Maximum concentration and area under the curve for 0 to infinity min for insulin were bioequivalent for the 32G × 4 needle relative to the 32G × 6 and the 31G × 8 needles.”
But that 
is not what we are talking about though, we are talking about speed.
“The time to the maximum insulin concentration was bioequivalent for the 32G × 4 needle relative to the 32G × 6 needle, but not the 31G × 8 needle.”
There is the statement that the speed is not equivalent. That refutes the whole argument.
The second cross-reference:
The study was done on patients and their BG was monitored for 3 hours while they remained lying down?!
"The absorption rates were followed continuously for 3 h with the patient in the supine position. "
That’s pretty far from real-world. Use the 4mm when you are going to remain lying down for 3 hours after your injection…
The third cross-reference:
“Injections were administered at an approximately 90-degree angle into a raised skinfold for the 8-mm injection depth and without raising a skinfold for the 5-mm injection depth.”
Doing it for one and not the other is not the same thing. Raising the skin effectively removes the depth advantage of the 8mm needle.
As a side note, the 3rd study used 28 people, and did it twice. That’s a total of only 56 injections they are using for their conclusion. That’s less than a week of injections for me.
What I find very comical, but somewhat sad, is that all the people who are clamoring to get Fiasp could basically get the same speed improvement by simply taking their current rapid - NovoLog, Humalog, or Apidra - and just inject it IM. That would essentially be as fast as the Fiasp studies have shown.
Most of the professional diabetics inject IM.
Have you ever tried the 8mm needles? Why not try it.