AutoComplete has been disabled in Word 2007 due to the enormous quantity of Building Blocks (the expanded successor to AutoText) that would otherwise cause AutoComplete to occur just about all the time.
To force completion after typing the first four characters, you’ll need to press the F3 key. This will work with any Building Block, not just AutoText. But, it will work only if there are no other entries that begin with the same four letters.
Some users are finding it useful to use AutoCorrect entries instead of AutoText entries as a work-around, and to purposely create short names that match what they were typing in Word 2003 and earlier. There is still no AutoComplete display, but typing any Word separator (space, period, comma, etc.) will trigger that automatic change from the short name into the expanded form.