Add the first and the last numbers and double that answer.
Make a chart with five columns. Put four consecutive numbers one in each cell of a row and their sum in the fifth row. Repeat for several other numbers and see what kiind of pattern there is.
n + (n + 1) + (n + 2) + (n + 3)
[n + (n + 3)] * 2
The same. Proof positive for all numbers.