The iFlash-Quad is a board, part of the iFlash lineup designed by Tarkan, which allows for four microSD cards to be inserted in place of the 1.8" hard disk drive in later full-size iPods.

Image of iFlash Quad

iFlash-Quad is the thinnest of all iFlash boards, allowing for higher-capacity batteries to be installed.

The iFlash-Quad retails for $42 USD.

Compatibility

iFlash-Quad
Can be inserted 4 microSD cards
Compatibility (iPod video and later)   Fully natively compatible
  6th generation "160 GB thick" iPods REQUIRE an HDD ribbon to be used with iFlash-Quad.
Compatibility (iPod (4th generation))   Compatible, with 4th Gen iPod to iFlash Converter.
Compatibility (iPod (3rd generation))   Compatible, with 4th Gen iPod to iFlash Converter.
Compatibility (iPod (2nd generation) and earlier   Incompatible.
Compatibility (iPod mini)   Incompatible.

Resources

Link to iFlash's webpage on the iFlash Quad: https://www.iflash.xyz/store/iflash-quad/