Sum to n terms is
Sn = n/2 ( a + {n-1}d ) where a is the first term in the series,and d is the common difference.
So the sum of the first 4 terms, S4 = 4/2 ( a + {4-1} d). You just need to plug in the values of a,which is the first term,and d,the common difference. If you know how the formula is found it's easier than trying to remember it.
