OR you could do this
$myArr = array(2 => 'three', 3 => 'four');
$newArr = array_pad(array(), 4, 'FILLED');
This gives your desired result BUT the ordering is a little wierd, because of the order they were added. Indexes are okay though and that is what you wanted.
Array (  => three  => four  => FILLED  => FILLED )
hope this helps