How to add a fourth equation in the Microsoft Word Equation Editor?

Just type the three equations and then, while still in the box for the last equation, press the Enter key. A new dotted-border box appears below. Works both on Word 2007 and on Word 2010.

In the Office 2010 version of the equation editor, after inserting a brace, with it selected, I can just right-click on one of its places and chose "Insert Equation Before" or "Insert Equation After".

If you find it a bit tricky to hit the little boxes with the I-beam mouse cursor, you may be able to do it with the keyboard: move the text cursor around with the arrow keys and then use the menu key (generally between the right Alt and Ctrl on 'Windows keyboards') to get the context menu for the item under the cursor.