In the end, it turns out that it is not a question of amount of chip RAM but of speed.
Turns out the reason why there is an 8Mbytes SIMM in there is because there were no 60ns 2Mbytes SIMM in existence. Pretty much all 2Mbytes SIMMs were 80ns or slower.
The official reply from Brian:
About the A4000 chipram. Even though I have an 8 meg module, workbench says that I only have 2mb. I may have done this to make the 'TurboAGA', my amiga chipset overclocking project, operate at such a high frequency. It is a 8mb, -6 module. Commodore's 2mb module was a -8 I believe.
I guess my memory is as small as the amiga's.
And with that the official specs for the machine now are:
-A4000 with 2mb chip ram, 16mb fast ram, Commodore 3640 over-clocked 33MHz 68040.
800mb IDE HD
GVP EGS True Color Retargetable Color System with Video card, 2mb ram, slightly overclocked.
Additional Cybergraphics drivers.
But I can attest to the fact that his 3640 is REALLY overclocked to 33MHz.