Difference between 1-bit, 4-bit and 8-bit SDIO
There are two modes of communicating with an SD card: SD mode (sometimes incorrectly called SDIO), and SPI mode (Serial Peripheral Interface).  (SDIO actually refers to a Secure Digital Input Output card which is a superset of the SD card spec, and supports various I/O devices in addition to memory.)

An SD card comes up by default in 1-bit SD mode, but can be changed into 4-bit mode after startup.  If necessary, the card can also be switched into SPI mode, which is always 1-bit wide.  The bus width in SD mode can be anywhere from 1 to 4-bits (see 6.2.1).  There isn't any 8-bit SD mode, because there aren't enough pins on the SD card to support it.  There is an 8-bit SD mode for MMC (MultiMediaCard) cards which have more pins.

I don't see where anyone would want to run in SD mode with less than four data lines, unless they were limited for I/O lines.  So let's forget about that.

Since both SD card mode and SPI mode can run at similar frequencies (up to 50 MHz for SD mode, and perhaps 40 MHz for SPI mode), the main difference is going to be in the bus width.  So you will roughly get 4 to 5 times the throughput using the 4-bit SD mode compared to the 1-bit SPI mode.