The N64 came with 4MB of RAM soldered on the board. With the Expansion Pak, you could add an additional 4MB of RAM to bring it to a total of 8MB. Was 8MB the maximum amount of RAM that the N64 could support or could a person in theory develop an Expansion Pak with more than 4MB of RAM? Why or why not?
In theory you certainly can, just by paging the RAM in the expansion pack. Basically you would connect a register to the last byte(s) of the 4MB address space and use it to hold high address bits of the address space. Whenever you want to use a different 4MB page, you would write the higher 8 bit of the new address range into this register. This is very simple to implement.
The harder part is to write a game to support memory paging and to take advantage of additional memory.