DFDB Recognition
Roux users often have trouble predicting the 3-cycle cases that occur in step 4c. After many solves you can wing it and mostly determine the 3-cycle by feel. But there can still be mistakes and solves where the correct AUF wasn't predicted. So around 2010 I started looking into solving the problem. In April of 2010 I developed the first ever recognition method for step 4c. It was based on the stickers that are at FU and BU if the corners are imagined to be aligned correctly after step 4b. I wasn't completely satisfied with this recognition method because I found the lookahead difficult. So in 2012 I developed a new type of recognition that is based on the U and D stickers of the M slice edges. Louis de Mendonça eventually started using the recognition and his promotion of it is likely the reason that it is now used by many Roux users.
DFDB
In this recognition method, the user tracks either the DF and DB stickers or the UF and UB stickers while completing step 4b. The U layer is aligned after 4b based on their position. It seems that most Roux users only track the DF + DB stickers, but the recognition method was developed for either UF + UB or DF + DB to be used. Or if the misoriented centers technique is used, the applicable U and D stickers in that situation. Since my original post and guide there have been many other guides created. Below is my original, but it is best to learn from one of the newer guides since they teach the newest ways of recognizing the 3-cycles. These guides can be found in the Roux Discord server or YouTube.
https://www.speedsolving.com/forum/showthread.php?35350-Roux-4b-to-4c-Transition
FUBU / BUFU
After I made a couple of simple posts about this recognition, Michał Ciebiad created a video explaining the recognition.
4c Recognition History
For more about the history of Roux 4c recognition methods, details are provided on the Method Library:
https://sites.google.com/view/methodlibrary/3x3-methods/roux