If you want to merge arrays, you should use jQuery.merge().
$.merge( [ 0, 1, 2 ], [ 2, 3, 4 ] );
//result is [ 0, 1, 2, 2, 3, 4 ]
You could do it:
var oldArray = [ obj1, obj2, obj3, obj4 ];
var newArray = [ obj5 ];
oModel.setData( $.merge( oldArray, newArray ) );
But, if you want just to add one item at the end of the array, you also could do it:
oModel.getData().push( obj5 );