The hadith is not a fabrication, but an authentic one. This is how the hadith goes. The man was married to a wife and after sometime, he went to the prophet, pbuh and complained that he is married, but poor, and the prophet advised him to marry another wife, he did, but still came to the prophet with the same complain and he got the same advice up to the time he married the 4th wife. Now, the point here is the poor state of the man. This man was not poor financially, on the contrary, he was a very wealthy man. The word used in Arabic for poor refers to lack of issue, that is child or children, which to them, no matter how rich you are, if you don't have a child or children, you are a poor man. So, when he married the 4th wife, that one got pregnant and the moment she delivered, the others started bearing children too. The 4th wife like now opened the door to the riches, that is fruits of the womb. May God bless us for the good we do and forgive us for our mistakes and limited knowledge.