Well I have been wondering last night til now and I think the pattern of there entire VSA-100 series would of been like the following:
3dfx Voodoo4 4000 AGP/PCI 16MB SDRAM 1x 16MB
3dfx Voodoo4 4500 AGP/PCI/PCI Mac 32MB SDRAM 1x 32MB
3dfx Voodoo4 4800 AGP/PCI 64MB SDRAM 1x 64MB
3dfx Voodoo5 5000 AGP/PCI 32MB SGRAM 2x 16MB
3dfx Voodoo5 5500 AGP/PCI/PCI Mac 64MB SDRAM 2x 32MB
3dfx Voodoo5 5800 AGP 128MB SDRAM 4x 32MB
It would of seemed a logical naming per class and since a Voodoo5 6000 is called a 6000 and not a 5800 seems more a mystery to me, since the high end Voodoo4 is the main number 4 with an 800 behind it.
Kinda obvious to some certain degree a Voodoo4 5000 would not been possible otherwise you would had an odd number for that card something like Voodoo5 5200 AGP/PCI maybe and the Voodoo4 4000 could of been a Voodoo4 4200 AGP/PCI instead by which you could of gotten this pattern, where by the best Voodoo5 could of kept it's current name.
3dfx Voodoo4 4200 AGP/PCI 16MB SDRAM 1x 16MB
3dfx Voodoo4 4500 AGP/PCI/PCI Mac 32MB SDRAM 1x 32MB
3dfx Voodoo4 5000 AGP/PCI 64MB SDRAM 1x 64MB
3dfx Voodoo5 5200 AGP/PCI 32MB SGRAM 2x 16MB
3dfx Voodoo5 5500 AGP/PCI/PCI Mac 64MB SDRAM 2x 32MB
3dfx Voodoo5 6000 AGP 128MB SDRAM 4x 32MB
Might of been possible, but still odd in a way hehe.