I'd highly recomend you give reverse sear a go. It definitely works best with thick steaks but a nice low oven temp 120C (250F) and pull when the steak is 40C (120F), then into a ripping hot pan with oil in it gives a fantastic sear.
I'm not suggesting it's better than your method or anything but well worth trying imo!
> why is it significant that we slice like a[i:i+4:i+4] rather than just a[i:i+4]?
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.