Well I had never seen that "full slice" expression syntax before. It turns out that it's important because it controls the capacity of the new slice. The capacity of the new slice is now i+4 - i.
So by using the full slice expression you get a slice of length 4 and capacity 4. Without doing this the capacity would be equal to the capacity of the original slice.
I suppose that by controlling the capacity that you eliminate the bounds check.
Edit: weird, this was supposed to be an update to a previous comment I made, but this is a different comment now