Well solutions have been given to you. Get 2 groups of 4x 512Mbit SDRAM chips with 8-bit data bus, and find someone that will torture themselves to route the PCB for it, if at all possible. If you have searched extensively for larger SDRAM chips and found 0 results, then its unlikely a 4Gbit SDR SDRAM chip exists, off the shelf, in 1 package, because there is no large scale demand for it. But it looks like you're pretty fixed to the parts chosen, so that also means the answer is pretty fixed.
Higher density memory chips have moved to DDR, DDR2 etc. buses which allow for reasonable access times to the whole memory array. At 512MB and a 100MHz SDR 32-bit bus (if you happen to get a working PCB layout for that), you're looking at 1.2+ seconds to do a full read of the array, neglecting access times, maximum burst sizes, memory refresh cycles and protocol/addressing overhead, which will probably increase it to 1.5 seconds or so. This is the reason for the "why?". I think most applications wanting 512MB would also want/need the memory bandwidth to support it.
If all you need is capacity, you could look in other ways of emulation. Multiple PSRAM chips that work over QuadSPI (note that ST's memory-mapped QuadSPI controller is meant for read-only modes only IIRC, e.g. FLASH). Or crazy idea: a 1TB SD card and trust the wear leveling. Even the most trash quality FLASH with only 300P/E cycles would last almost 1yr when it's continuously written at 12.5MB/s (SDIO bus 50MHz).