If I'm correct within the Tenach it's always the term Elohim following the term YHVH. You always read verses and phrases stating: 'YHVH Elohim' and never 'Elohim YHVH'. So why is this?
It's not always like this as Double AA points out.
The reason why you would want to precede them in that order is hinted to by Rashi on the first verse in Bereishis -- Elokim is connected to Judgment and 4-letter Name - to Mercy, and Hashem precedes Mercy to Judgment, hence the names expressing this precedence come in that order.