Sequentially chain your callbacks in jQuery - Two ways

$("#vehicle1").fadeIn("slow", function(){
  $("#vehicle2").fadeIn("slow", function(){
    $("#vehicle3").fadeIn("slow", function(){
      $("#vehicle4").fadeIn("slow");
    });
  });
});

(function showVehicle(elem){
  elem.fadeIn('slow',function(){
    $(this).next().length && showVehicle($(this).next()); 
  });
})( $("div.vehicle:first") );

$('div.vehicle')
  .bind('showVehicle',function(e) {$(this).fadeIn('slow',function(){ 
    $(this).next().length && $(this).next().trigger("showVehicle"); 
  })})
  .eq(0)
    .trigger('showVehicle');