The 8 MB barrier is the ZII adressspace ZIII can adress around 4GB. Still you can only add pure ZII cards to a total max of 8MB, but ZII/ZIII cards allocate their memory in ZIII adresspace so they don`t interfere.
But why onearth do you want to add RAM in form of deadslow ZII RAM expansion to an A4000 ? There are dozens of better ways to expand the RAM on such a system.
