If your model number starts with G4+MM, you have a Matrox G400 MAX (which has the core running at 150MHz, the RAM at 200 and the RAMDAC at 360MHz). If the model number starts with G4+M, you have a 'vanilla' G400, which core runs at 125MHz, it's memory at 166Mhz and the RAMDAC at 300MHz
The next part can either be DH, which stands for Dualhead, or SH, i.e. Singlehead
4A means that your G400 is capable of running it's AGP texture transfer speed at up to 4x, 4 means it can handle up to 2x AGP
32 or 16 is the number of megabytes of RAM on the card
G means it is equipped with SGRAM, DG means it is equipped with SDRAM

So you G4+M4A32DG can run under AGP 4x....