[{_id:1,prop1:"val1",prop2:"val2",prop3:[{title:"title1",arr:["val1","val2","val3"],},{title:"title2",arr:["val1","val2","val3"],},{title:"title3",arr:["val1","val2","val3"],/**only update this array*/}]}]

db.collection.update({"_id":1,"prop3.title":"title3"},{"$push":{"prop3.$.arr":"val4"}})