Laravel 4如何合并数据库表并通过created_at对它们进行排序？

I have 4 database tables that I want to merge together, and than I want to order all the data by created_at field. With my code all data is still grouped per database table, the result should be some sort of timeline of all my tables data. What am I doing wrong? It is only showing 4 items...I have 4 database tables that I want to merge t