PSP developer Mathieulh has announced on the QJ.NET PSP Development forums that Dark AleX has released update 4 for the 3.71 M33 PlayStation Portable custom firmware. Here's the changelog for 3.71 M33-4:

* Bugfix: Multidisc psx games bought in the psn store would not work due to the format being different and M33 treating them as decrypted games.
* Added support for decrypted multidisc psx games created with popstation_md (supplied with this release).

This update also includes the multi-disc Popsloader Dark AleX has been working on. Here's the changelog for the new Popsloader:

* Bugfix: Games without compression had sound issues when using 3.72 pops. Fixed.
* Added support for multidisc px games created with popstation_md.
o Note: multi disc games will only work in 3.71 and 3.72 pops. Previous pops will exit to the xmb with generic error 0x80000004 when attempting to run multidisc games.

All you need to enable update 4 is a previous version of 3.71 M33 on your PSP. There's more info in the readme file included in the download.