At HW level, there is no issue to use 4-bit eMMC as BootROM first boot level only use one data line, e.g. CLK/CMD and D0 on PE3/PG6 and PB14 (see AN5031).
To restrict usage to 4-bit eMMC in TF-A and uBoot/Linux, you should adapt the Device Tree.
Please note that we currently have a limitation for eMMC boot from Micron devices, see ES0438. Meanwhile you should use Toshiba or Kingston devices. This will be solved in future silicon.
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