For others looking, on a non-Lite version of CM4 (so one that has eMMC storage), using
dtoverlay=sdio instead of
dtoverlay=sdhost worked for me. Additional info:
dtoverlay -h sdio
Info: Selects the bcm2835-sdhost SD/MMC driver, optionally with overclock,
and enables SDIO via GPIOs 22-27. An example of use in 1-bit mode is
dtoverlay -h sdhost
Info: Selects the bcm2835-sdhost SD/MMC driver, optionally with overclock.
N.B. This overlay is designed for situations where the mmc driver is
the default, so it disables the other (mmc) interface - this will kill
WLAN on a Pi3. If this isn't what you want, either use the sdtweak
overlay or the new sd_* dtparams of the base DTBs.