If the addressing is limited to 4mb, why do they ship the board with an 8mb part? Or is the extra 4mb only accessible using the himem library?

Hi all, I just purchased a ESP-WROVER-KIT-VB boards, and just discovered the is only comes with 8MB of the 'PSRAM' on the board. That memory is accessed via the SPI bus, right? Are there any expansion modules available for the board which would provide a way to get more PSRAM, either via SPI or anot...