I found a deal on a 6004 holster when I bought my M9 a few weeks ago.
I normally use the 6378, but the price on this was too good to pass up, and all I wanted it for was to attach it to my spare mid-ride UBL and run my riggers belt through it for a quick, Bump In The Night lightweight setup.
I gotta say, for me - and opinions vary like assholes - but as soon as I got it all set up and tried it i out, I hated the 6004. It stuck much too far out compared to the 6378 and mounts weirdly to the UBL. Almost offset if you can imagine - the bottom of the holster really dug into my leg.
Now, I am looking for a deal on a FDE 6378 for an M9 to replace the 6004.