Assuming the first row of data is in A1:D1,
put this in E1

=AVERAGE(A1:D1)
press enter and then select the cell again.
Notice that little square in the lower right corner? That's called the Fill handle.
Hover over the Fill handle and your mouse will turn into a black plus sign.
Double click and the formula will fill down as far as the data in the column to the left goes.