What I noticed is that the eMMC to uSD adapter does not work on the Rock Pi 4 itself. Get the following in dmesg:
[ 872.743749] vcc_sd: unsupportable voltage range: 3300000-3000000uV
[ 872.744374] rockchip-iodomain ff770000.syscon:io-domains: Setting to 3000000 done
[ 872.745055] rockchip-iodomain ff770000.syscon:io-domains: Setting to 3000000 done
[ 872.785761] vcc_sd: unsupportable voltage range: 3300000-3000000uV
[ 872.786387] rockchip-iodomain ff770000.syscon:io-domains: Setting to 3000000 done
[ 872.787067] rockchip-iodomain ff770000.syscon:io-domains: Setting to 3000000 done
[ 872.803764] mmc_host mmc0: Bus speed (slot 0) = 300000Hz (slot req 300000Hz, actual 300000HZ div = 0)
[ 872.830857] vcc_sd: unsupportable voltage range: 3300000-3000000uV
[ 872.831558] rockchip-iodomain ff770000.syscon:io-domains: Setting to 3000000 done
[ 872.832237] rockchip-iodomain ff770000.syscon:io-domains: Setting to 3000000 done
[ 872.847735] mmc_host mmc0: Bus speed (slot 0) = 200000Hz (slot req 200000Hz, actual 200000HZ div = 0)
[ 872.877446] vcc_sd: unsupportable voltage range: 3300000-3000000uV
[ 872.878053] rockchip-iodomain ff770000.syscon:io-domains: Setting to 3000000 done
[ 872.878731] rockchip-iodomain ff770000.syscon:io-domains: Setting to 3000000 done
[ 872.895696] mmc_host mmc0: Bus speed (slot 0) = 100000Hz (slot req 100000Hz, actual 100000HZ div = 0)
It works on my laptop though running Linux:
[108382.264655] mmc0: new high speed MMC card at address 0001
[108382.265548] mmcblk0: mmc0:0001 SOD16G 14.6 GiB
[108382.265723] mmcblk0boot0: mmc0:0001 SOD16G partition 1 4.00 MiB
[108382.265877] mmcblk0boot1: mmc0:0001 SOD16G partition 2 4.00 MiB
[108382.277936] mmcblk0: p1 p2 p3 p4 p5
[108424.072301] mmc0: card 0001 removed
[108465.766913] mmc0: new high speed MMC card at address 0001
[108465.767144] mmcblk0: mmc0:0001 SOD16G 14.6 GiB
[108465.767262] mmcblk0boot0: mmc0:0001 SOD16G partition 1 4.00 MiB
[108465.767371] mmcblk0boot1: mmc0:0001 SOD16G partition 2 4.00 MiB
[108465.778398] mmcblk0: p1 p2 p3 p4 p5