The title says "the mean of four variables IN A ROW," so I assume you want to use the m[,:] subscript reduction operator.
The easiest way is to just extract the four variables and use the ':' operator to compute the means:
A = m[ , 1:4]; /* extract four columns */
f = A[ , :]; /* mean of four columns */
Alternatively, If you don't need the e variable, you can define varNames = "a":"d" and then directly compute f=m[,:].