@crackpot: Some devices may tolerate a lower voltage than the full USB 5v. The kindle battery supplies nearly 4v.
You can get miniPCIe breakout boards with integrated 4-port USB.
Why do you need 4 microSDHC readers? You can get microSDHC cards up to 64GB, although full-size SDHC cards cost less, and could still fit in the 3G modem space if careful. You could even remove part of the plastic shell on an SDHC card to make it smaller.
Although slower, you can use SPI to talk to SDHC cards as well (as mentioned above), and this will not interfere with a 3G modem. Not all microSDHC cards are guaranteed to support SPI, but full-size SDHC cards are required to have SPI support. You can use software "bit-bashed" SPI drivers on "re-used" GPIO lines (such as power LEDs) if the onboard SPI bus is not easy to hijack for your own personal use.
These days, rather than using a floppy connecter (no longer as common as back in 2006), I would instead try to find a microSDHC card that supports SPI, and use the microSD adapter that comes with most of them as a socket by soldering directly to the outer pads on the adapter.
"SD format" cards larger than 64GB are REQUIRED to NOT support SDHC protocol, even though they use identical hardware, forcing end-users to buy all new equipment (for pure greedy political reasons)!
Instead, "SD" cards larger than 64GB must use the new XD software protocol, which would require new drivers in the firmware. You would have to port (or write) a linux XD card driver to use in the kindles, if you want to go larger than 64GB.