first 4 and last 4 items in the iterable

a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> # What we want is [0, 1, 2, 3] and [6, 7, 8, 9]

>>> # Now the first part isn't that bad.
... # We can get an element pretty easy
... a[0]
0
>>> a[1]
1
>>> a[2]
2
>>> a[0], a[1], a[2]
(0, 1, 2)
>>> # Now there's a shorthand for grabbing a "slice" of an iterable:
... a[0:4]
[0, 1, 2, 3]

>>> # This grabbed elements a[0] up through a[3]
... # Notice 4 is one-greater than the last element
... # You've probably noticed this is pretty common
... # in anything that iterates.
...
>>> # There's another trick with slices.
... # If you leave off one or the other of the numbers, it "slices"
... # from the beginning (no left number [  :3]) 
>>> a[  :3]
[0, 1, 2]
>>> # or stops slicing
... # with the end (no right number [3:  ])
>>> a[3:  ]
[3, 4, 5, 6, 7, 8, 9]

>>> # So our first part is done
... # We want the first four elements
... # Those are elements 0-3
... # So we just take a slice up to (not including)
... # element 4:
... a[:4]
[0, 1, 2, 3]
>>>
>>> # Now the last part is kind of done if we know how many
... # elements are in the iterable and don't mind some arithmetic.
... # However there's an easier (and more elegant) way:
... # Lets look at our list again:
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# Now watch this:
... a[-1]
9
>>> a[9]
9
# See both a[9] and a[-1] point to the last item in our list...
>>> a[-2]
8
>>> a[-3]
7
>>> # As you can see, negative indices are counted backwards
... # from the end of the iterable; starting with -1
... # as the last element.
... # So what we want for the last 4 elements is:
... a[-4], a[-3], a[-2], a[-1]
(6, 7, 8, 9)

>>> #  Using slice notation, we use the "lower"
... # index first like usual; but since going one index
... # higher than -1 (0) wont work here we'll use the no-upper-bound
... # trick and leave off the second index in the slice [-4:  ] 
... # so the slice will take elements up to and including the last one:

... a[-4: ]
[6, 7, 8, 9]

>>> # So our first four and last four elements are:
... a[:4] + a[-4:]
[0, 1, 2, 3, 6, 7, 8, 9]