The Pi 4 uses DRAM with on-die ECC, which AFAIK does not provide any means of reporting errors (corrected or uncorrected) to the SoC's memory controller. It is

The Pi 4 uses DRAM with on-die ECC, which AFAIK does not provide any means of reporting errors (corrected or uncorrected) to the SoC's memory controller. It is effectively a cost-saving measure to improve DRAM yields. As such, it does little to guarantee that there are no memory errors.