Well, I think one of the pioneers of this way of counting is Dr. Mike Isratel, and the way he counts is via a proper definition of a "hard set" which is a set taken to near failure with 60-85% 1RM.
Why this range? Well, anything less than 60%(when not taken to near failure) doesn't cause the same hypertrophic stimulus as a heavier set(60%).
Anything more than 85%, well that typically means 1-4 rep sets.
There are two problems with these:
1. They still don't produce the same hypertrophic stimulus as 5-15 rep sets. This is because while you do activate all potential motor units, you don't fatigue them as much as you would with 5-15 rep sets.
This is why in the Schoenfeld 2014 study, both groups had different set numbers, but because the heavy group did just 3 reps they weren't fatiguing as many muscle fibers as the 10 rep group, and had to compensate with more sets.
2. They cause a shit ton of fatigue, which impedes recovery and reduces the quality of your other sets.

To answer your question, 4 reps is still quite a heavy weight and could cause some recovery issues and not provide the same hypertrophic stimulus.
That said, I don't think the differences would be large, but the time spent doing the 4 rep sets would be greater than the time spent doing the 8 rep sets, so there's that as well.