Originally Posted by beanbaggins
If I am not mistaken, I believe that this will let you play DLC without having to use reactpsn.
No sorry you are mistaken.
reactPSN needs RAP files to work. These can be created on CFW 3.55 by reactPSN from the rif files that are present in your exdata folder. i.e. it performs a RIF2RAP for you. Problem is reactPSN will not do this on CFW 4.xx, hence you need to use this program to do it for you.
More information so that you can understand this if you are interested:
In order for reactPSN to work the PS3 needs to be able to decrypt locally licensed EDAT files. In order to do this it needs a number of keys, the main 2 that we are interested in are the devKLic and the keyFromRif. The devKLic is either a known hard coded key in the F/W this is the case for PSX, PS2, PSP, minis, Demos and others. For DLC, updates, PSN games this key is in the application executable (e.g. BOOT.BIN). The other key the keyFromRif is what the RAP file provides. The other 2 keys it needs are the EDATKEY and the EDATHASH, there are <V4 versions of theses and >=V4 versions of these. I have tired to keep this simple, hope it helps.
EDIT: It is also needed as you say to make games etc work with out reactPSN. In order to convert an EDAT file from locally licensed to free the same keys are needed as described above.