Simplify the expression first before determining how many terms there are. This means you should combine like terms.
8x - 2y + y - 4
Both y's can be combined. If you have -2 y's and add 1 y, you have -1 y's left.
8x - y - 4
Terms are separated by addition or subtraction signs. There are THREE terms here: 8x, -y, and -4.