Allegro 4 is the classic library, whose API is backwards compatible all the way back to Allegro 2.0 for DOS/DJGPP (1996). This version of Allegro is no longer actively developed, but we still apply patches sent to us by contributors, mainly to fix minor bugs. Old applications that were written using Allegro 4 may continue to do so, although it might be a good idea to start Porting from A4 to A5 if possible. For new applications, the use of Allegro 4 is discouraged. Please use Allegro 5 to develop new programs. The term "Allegro 4" is often used to describe anything prior to Allegro 5.
Through the lifespan leading up through Allegro 4, users have contributed several library extensions to handle things like scrolling tile maps and import and export of various file formats (e.g. PNG, GIF, JPEG images, MPEG video, Ogg, MP3, IT, S3M, XM music, TTF fonts, and more). There are also bindings for several programming languages available, such as Python, Perl, Scheme, C#, D and others.
Allegro 4 can be used in conjunction with OpenGL by using the library AllegroGL which extends Allegro's functionality into OpenGL and therefore the hardware.
For hardware accelerated 3D and 2D acceleration on Linux, Mac OS X and DOS, AllegroGL and OpenLayer are available. They are two add-on libraries that use OpenGL for accelerated graphics routines and use Allegro for all other gaming needs. Note that, combined with Glide and MesaFX (using 3dfx hardware), AllegroGL is one of the few available opensource solutions for hardware accelerated 3D under DOS.
Pre-compiled binaries and source code are available for most platforms and can be downloaded at the allegro.cc files section. As of 03/28/2011, the latest version available at the allegro.cc files page is version 4.2.3, but the latest version of Allegro 4 is Allegro 18.104.22.168, the source code of which is available at the sourceforge download page.
Compiling Allegro 4 from source is covered here: Build