Simple. Just look at the values in binary:
1 == 00000001
2 == 00000010
3 == 00000011
4 == 00000100
5 == 00000101
When you use a shift operator, you move the binary value "n" bits to the left or right.
So "1" left shifted 2 places:
0001 becomes 0100
1 " 8
And "4" right shifted 4 places:
0100 Becomes 0000
4 " 0
because all the "1" bits are shifted out of teh value and discarded.
A "shift" is just that - it does not "wrap" bits back to teh top or bottom of the value.