What the description tells you is:
- When a target dies, it will spread up to 4 stacks on nearby enemies
- The stacks on the spread targets cannot exceed the stacks on the dying target
What the description doesn’t tell you is:
- That spread radius is 5m
- The stacks have to be 2 or more on the dying enemy in order to spread (another bug report already exists)
- The stacks on the spread targets cannot exceed 4 in any case
That is, let us have enemy A with 8 stacks and a nearby enemy B with 6 stacks. Enemy A dies. Based on the description, it is expected that enemy B will get 2 stacks and bumped to 8 stacks total. A can spread 4 stacks but the stack count on B cannot exceed 8 so only 2 stacks speed. What happens in reality is nothing because B has more than 4 stacks, even the duration is not refreshed.