Is it possible to use 4-pin mode for an SD card with a microcontroller instead of SPI mode?
That's what I've found:


  The SD 1-bit protocol is a synchronous
  serial protocol with one data line,
  used for bulk data transfers, one
  clock line for synchronization, and
  one command line, used for sending
  command frames. The SD 1-bit protocol
  explicitly supports bus sharing. A
  simple single-master arbitration
  scheme allows multiple SD cards to
  share a single clock and DAT0 line.
  The SD 4-bit protocol is nearly
  identical to the SD 1-bit protocol.
  The main dierence is the bus width 
  bulk data transfers occur over a 4-bit
  parallel bus instead of a single wire.
  With proper design, this has the
  potential to quadruple the throughput
  for bulk data transfers. Both the SD
  1-bit and 4-bit protocols by default
  require CRC protection of bulk data
  transfers. A CRC, or Cyclic Redundancy
  Check, is a simple method for
  detecting the presence of simple
  bit-inversion errors in a transmitted
  block of data. In SD 4-bit mode, the
  input data is multiplexed over the
  four bus (DAT) lines and the 16-bit
  CRC is calculated independently for
  each of the four lines. In an
  all-software implementation,
  calculating the CRC under these
  conditions can be so complex that the
  computational overhead may mitigate
  the benets of the wider 4-bit bus. A
  4-bit parallel CRC is trivial to
  implement in hardware, however, so
  custom ASIC or programmable-logic
  solutions are more likely to benet
  from the wider bus.


http://alumni.cs.ucr.edu/~amitra/sdcard/Additional/sdcard_appnote_foust.pdf

I believe you wouldn't have enough power to calculate crc32 in 4-bit mode. But without CRC it might be possible.