Debian does not recognise RAM although BIOS does

My debian system seems not to use the full ram which is installed (6GB) in my mini-itx. Here is some output of dmesg, free and dmidecode that might help. In BIOS, I can see both (2GB + 4GB).

root@debian-simon:/home/simon# dmesg | grep -i memory
[    0.016839] initial memory mapped: [mem 0x00000000-0  x1cffffff]
[    0.017385] Early memory node ranges
[    0.038097] Reserving Intel graphics memory at [mem 0x60000000-0x7fffffff]
[    0.038453] PM: Registered nosave memory: [mem 0x00000000-0x00000fff]
[    0.038457] PM: Registered nosave memory: [mem 0x0009b000-0x0009bfff]
[    0.038458] PM: Registered nosave memory: [mem 0x0009c000-0x0009ffff]
[    0.038460] PM: Registered nosave memory: [mem 0x000a0000-0x000dffff]
[    0.038462] PM: Registered nosave memory: [mem 0x000e0000-0x000fffff]
[    0.038465] PM: Registered nosave memory: [mem 0x10000000-0x12150fff]
[    0.098144] Memory: 1421780K/1480704K available (6630K kernel code, 639K rwdata, 2076K rodata, 840K init, 436K bss, 58924K reserved, 0K cma-reserved, 618212K highmem)
[    0.098153] virtual kernel memory layout:
[    0.154868] Freeing SMP alternatives memory: 24K
[    1.436131] Freeing initrd memory: 31204K
[    3.391608] Freeing unused kernel image memory: 840K

root@debian-simon:/home/simon# free
              total        used        free      shared  buff/cache   available
Mem:        1453848      808424      191980       36952      453444      429164
Swap:       7771128      215552     7555576

root@debian-simon:/home/simon#  sudo dmidecode | sed -n '/^Memory Device$/,/^$/p'
Memory Device
        Array Handle: 0x000E
        Error Information Handle: Not Provided
        Total Width: 64 bits
        Data Width: 64 bits
        Size: 4096 MB
        Form Factor: SODIMM
        Set: None
        Locator: ChannelA-DIMM0
        Bank Locator: BANK 0
        Type: DDR3
        Type Detail: Synchronous
        Speed: 1333 MT/s
        Manufacturer: Samsung
        Serial Number: 67422D51
        Asset Tag: 9876543210  [Titel anhand dieser ISBN in Citavi-Projekt übernehmen] 
        Part Number: M471B5273DH0-CH9
        Rank: 2
        Configured Memory Speed: 1333 MT/s
        Minimum Voltage: 1.5 V
        Maximum Voltage: 1.5 V
        Configured Voltage: 1.5 V

Memory Device
        Array Handle: 0x000E
        Error Information Handle: Not Provided
        Total Width: 64 bits
        Data Width: 64 bits
        Size: 2048 MB
        Form Factor: SODIMM
        Set: None
        Locator: ChannelA-DIMM1
        Bank Locator: BANK 1
        Type: DDR3
        Type Detail: Synchronous
        Speed: 1333 MT/s
        Manufacturer: Unde
        Serial Number: 422FD906
        Asset Tag: 9876543210
        Part Number: RMT3020EF48E8W1333
        Rank: 2
        Configured Memory Speed: 1333 MT/s
        Minimum Voltage: 1.5 V
        Maximum Voltage: 1.5 V
        Configured Voltage: 1.5 V

*+++++++ Please ignore this part ++++++++++ The stackoverflow system wants me to post more information instead of code, but I honestly dont know whatt to add here.