Only 16GB of 20GB memory / RAM : found by kernel

free -g reports 17GB

ROM set reports 20GB as does the below (4+16GB)   How to get the 4GB
bank to usable

kernel 5.10.x

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dmidecode -t memory

# dmidecode 3.3
Getting SMBIOS data from sysfs.
SMBIOS 3.2.1 present.

Handle 0x000A, DMI type 16, 23 bytes
Physical Memory Array
    Location: System Board Or Motherboard
    Use: System Memory
    Error Correction Type: None
    Maximum Capacity: 32 GB
    Error Information Handle: 0x0009
    Number Of Devices: 2

Handle 0x0011, DMI type 17, 40 bytes
Memory Device
    Array Handle: 0x000A
    Error Information Handle: 0x0010
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 4 GB
    Form Factor: SODIMM
    Set: None
    Locator: DIMM 0
    Bank Locator: P0 CHANNEL A
    Type: DDR4
    Type Detail: Synchronous Unbuffered (Unregistered)
    Speed: 3200 MT/s
    Manufacturer: Hynix
    Serial Number: 00000000
    Asset Tag: Not Specified
    Part Number: HMA851S6CJR6N-XN
    Rank: 1
    Configured Memory Speed: 2400 MT/s
    Minimum Voltage: 1.2 V
    Maximum Voltage: 1.2 V
    Configured Voltage: 1.2 V

Handle 0x0014, DMI type 17, 40 bytes
Memory Device
    Array Handle: 0x000A
    Error Information Handle: 0x0013
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 16 GB
    Form Factor: SODIMM
    Set: None
    Locator: DIMM 0
    Bank Locator: P0 CHANNEL B
    Type: DDR4
    Type Detail: Synchronous Unbuffered (Unregistered)
    Speed: 2667 MT/s
    Manufacturer: A-DATA Technology
    Serial Number: 00870000
    Asset Tag: Not Specified
    Part Number:
    Rank: 1
    Configured Memory Speed: 2400 MT/s
    Minimum Voltage: 1.2 V
    Maximum Voltage: 1.2 V
    Configured Voltage: 1.2 V