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.
As of 06/09/2018, the latest version of allegro 4 is 4.4.3. To use allegro 4 you need to download binaries compatible with your compiler, or to build them yourself from source code.
Older versions of Allegro 4.2.3 and Allegro 4.4.2 are available at the allegro.cc files page. However, these are for legacy purposes only at this point due to the compilers used to build them being very out of date.
There are no longer official builds of Allegro 4, as it is no longer under active development. It is unlikely a new version will be released. You are advised to use the new hardware accelerated version of allegro, which is Allegro 5.
Edgar Reynaldo provides modern up to date versions of Allegro 4.4.3 hosted on bitbucket.
You can get older versions of allegro from liballeg.org. Find them here : https://liballeg.org/old.html
To get the latest version of Allegro 4 you need to use GIT. See instructions here : https://liballeg.org/git.html