Actually the problem might just be that the Linux kernel doesn't
understand extended vendor-specific display timings. I've decoded your
monitor's EDID and it reveals that 3840x2160 30Hz is mentioned:

edid-decode (hex):

00 ff ff ff ff ff ff 00 04 69 a3 28 24 51 00 00 
0e 1b 01 03 80 3e 22 78 3a 1c b5 a3 57 4f a0 27 
0d 50 54 bf ef 00 d1 c0 81 40 81 80 95 00 b3 00 
71 4f 81 c0 01 01 04 74 00 30 f2 70 5a 80 b0 58 
8a 00 6d 55 21 00 00 1a 02 3a 80 18 71 38 2d 40 
58 2c 45 00 6d 55 21 00 00 1e 00 00 00 fd 00 1e 
50 18 a0 1e 00 0a 20 20 20 20 20 20 00 00 00 fc 
00 41 53 55 53 20 50 42 32 38 37 51 0a 20 01 83 

02 03 2c f1 51 01 02 03 11 12 13 04 14 05 0e 0f 
1d 1e 1f 90 20 22 23 09 17 07 83 01 00 00 6d 03 
0c 00 20 00 00 3c 20 00 60 01 02 03 56 5e 00 a0 
a0 a0 29 50 30 20 35 00 6d 55 21 00 00 1e e2 68 
00 a0 a0 40 2e 60 30 20 36 00 6d 55 21 00 00 1a 
01 1d 00 bc 52 d0 1e 20 b8 28 55 40 6d 55 21 00 
00 1e 8c 0a d0 90 20 40 31 20 0c 40 55 00 6d 55 
21 00 00 18 00 00 00 00 00 00 00 00 00 00 00 5e 

----------------

EDID version: 1.3
Manufacturer: ACI Model 10403 Serial Number 20772
Made in week 14 of 2017
Digital display
Maximum image size: 62 cm x 34 cm
Gamma: 2.20
DPMS levels: Off
Undefined display color type
First detailed timing is preferred timing
Color Characteristics
  Red:   0.6367, 0.3408
  Green: 0.3115, 0.6250
  Blue:  0.1542, 0.0537
  White: 0.3134, 0.3291
Established Timings I & II
    720x400    70.082 Hz   9:5    31.467 kHz  28.320 MHz (IBM)
    640x480    59.940 Hz   4:3    31.469 kHz  25.175 MHz (DMT)
    640x480    66.667 Hz   4:3    35.000 kHz  30.240 MHz (Apple)
    640x480    72.809 Hz   4:3    37.861 kHz  31.500 MHz (DMT)
    640x480    75.000 Hz   4:3    37.500 kHz  31.500 MHz (DMT)
    800x600    56.250 Hz   4:3    35.156 kHz  36.000 MHz (DMT)
    800x600    60.317 Hz   4:3    37.879 kHz  40.000 MHz (DMT)
    800x600    72.188 Hz   4:3    48.077 kHz  50.000 MHz (DMT)
    800x600    75.000 Hz   4:3    46.875 kHz  49.500 MHz (DMT)
    832x624    74.551 Hz   4:3    49.726 kHz  57.284 MHz (Apple)
   1024x768    60.004 Hz   4:3    48.363 kHz  65.000 MHz (DMT)
   1024x768    70.069 Hz   4:3    56.476 kHz  75.000 MHz (DMT)
   1024x768    75.029 Hz   4:3    60.023 kHz  78.750 MHz (DMT)
   1280x1024   75.025 Hz   5:4    79.976 kHz 135.000 MHz (DMT)
Standard Timings
   1920x1080   60.000 Hz  16:9    67.500 kHz 148.500 MHz (DMT)
   1280x960    60.000 Hz   4:3    60.000 kHz 108.000 MHz (DMT)
   1280x1024   60.020 Hz   5:4    63.981 kHz 108.000 MHz (DMT)
   1440x900    59.887 Hz  16:10   55.935 kHz 106.500 MHz (DMT)
   1680x1050   59.954 Hz  16:10   65.290 kHz 146.250 MHz (DMT)
   1152x864    75.000 Hz   4:3    67.500 kHz 108.000 MHz (DMT)
   1280x720    60.000 Hz  16:9    45.000 kHz  74.250 MHz (DMT)
Detailed mode: Clock 297.000 MHz, 621 mm x 341 mm
               3840 4016 4104 4400 (176  88 296)
               2160 2168 2178 2250 (  8  10  72)
               +hsync -vsync
               VertFreq: 30.000 Hz, HorFreq: 67.500 kHz
Detailed mode: Clock 148.500 MHz, 621 mm x 341 mm
               1920 2008 2052 2200 ( 88  44 148)
               1080 1084 1089 1125 (  4   5  36)
               +hsync +vsync
               VertFreq: 60.000 Hz, HorFreq: 67.500 kHz
Display Range Limits
  Monitor ranges (GTF): 30-80 Hz V, 24-160 kHz H, max dotclock 300 MHz
Display Product Name: ASUS PB287Q
Has 1 extension block
Checksum: 0x83

----------------

CTA-861 Extension Block Revision 3
Underscans PC formats by default
Basic audio support
Supports YCbCr 4:4:4
Supports YCbCr 4:2:2
1 native detailed modes
40 bytes of CTA data blocks
  Video Data Block
      640x480    59.940 Hz   4:3    31.469 kHz  25.175 MHz (VIC   1)
      720x480    59.940 Hz   4:3    31.469 kHz  27.000 MHz (VIC   2)
      720x480    59.940 Hz  16:9    31.469 kHz  27.000 MHz (VIC   3)
      720x576    50.000 Hz   4:3    31.250 kHz  27.000 MHz (VIC  17)
      720x576    50.000 Hz  16:9    31.250 kHz  27.000 MHz (VIC  18)
     1280x720    50.000 Hz  16:9    37.500 kHz  74.250 MHz (VIC  19)
     1280x720    60.000 Hz  16:9    45.000 kHz  74.250 MHz (VIC   4)
     1920x1080i  50.000 Hz  16:9    28.125 kHz  74.250 MHz (VIC  20)
     1920x1080i  60.000 Hz  16:9    33.750 kHz  74.250 MHz (VIC   5)
     1440x480    59.940 Hz   4:3    31.469 kHz  54.000 MHz (VIC  14)
     1440x480    59.940 Hz  16:9    31.469 kHz  54.000 MHz (VIC  15)
     1440x576    50.000 Hz   4:3    31.250 kHz  54.000 MHz (VIC  29)
     1440x576    50.000 Hz  16:9    31.250 kHz  54.000 MHz (VIC  30)
     1920x1080   50.000 Hz  16:9    56.250 kHz 148.500 MHz (VIC  31)
     1920x1080   60.000 Hz  16:9    67.500 kHz 148.500 MHz (VIC  16, native)
     1920x1080   24.000 Hz  16:9    27.000 kHz  74.250 MHz (VIC  32)
     1920x1080   30.000 Hz  16:9    33.750 kHz  74.250 MHz (VIC  34)
  Audio Data Block
    Linear PCM, max channels 2
      Supported sample rates (kHz): 96 48 44.1 32
      Supported sample sizes (bits): 24 20 16
  Speaker Allocation Data Block
    Speaker map:
      FL/FR - Front Left/Right
  Vendor-Specific Data Block, OUI 0x000c03 (HDMI)
    Source physical address 2.0.0.0
    Maximum TMDS clock: 300 MHz
    Extended HDMI video details:
      HDMI VICs:
         3840x2160   30.000 Hz  16:9    67.500 kHz 297.000 MHz (HDMI VIC 1)
         3840x2160   25.000 Hz  16:9    56.250 kHz 297.000 MHz (HDMI VIC 2)
         3840x2160   24.000 Hz  16:9    54.000 kHz 297.000 MHz (HDMI VIC 3)
Detailed mode: Clock 241.500 MHz, 621 mm x 341 mm
               2560 2608 2640 2720 ( 48  32  80)
               1440 1443 1448 1481 (  3   5  33)
               +hsync +vsync
               VertFreq: 59.951 Hz, HorFreq: 88.787 kHz
Detailed mode: Clock 268.500 MHz, 621 mm x 341 mm
               2560 2608 2640 2720 ( 48  32  80)
               1600 1603 1609 1646 (  3   6  37)
               +hsync -vsync
               VertFreq: 59.972 Hz, HorFreq: 98.713 kHz
Detailed mode: Clock 74.250 MHz, 621 mm x 341 mm
               1280 1720 1760 1980 (440  40 220)
                720  725  730  750 (  5   5  20)
               +hsync +vsync
               VertFreq: 50.000 Hz, HorFreq: 37.500 kHz
Detailed mode: Clock 27.000 MHz, 621 mm x 341 mm
                720  732  796  864 ( 12  64  68)
                576  581  586  625 (  5   5  39)
               -hsync -vsync
               VertFreq: 50.000 Hz, HorFreq: 31.250 kHz
Checksum: 0x5e

** Package changed: mutter (Ubuntu) => xorg-server (Ubuntu)

** Also affects: linux (Ubuntu)
   Importance: Undecided
       Status: New

** Summary changed:

- External monitor will not display to 4k
+ i915 does not expose vendor-specific extended HDMI timings/modes

** Changed in: xorg-server (Ubuntu)
       Status: Incomplete => New

-- 
You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux in Ubuntu.
https://bugs.launchpad.net/bugs/1890418

Title:
  i915 does not expose vendor-specific extended HDMI timings/modes

Status in linux package in Ubuntu:
  Incomplete
Status in xorg-server package in Ubuntu:
  Incomplete

Bug description:
  I have a Toshiba radius 11 laptop which has a 4 k native screen. I
  have connected an Asus PB287Q 28" external monitor
  (https://www.asus.com/us/Commercial-Monitors/PB287Q/specifications/)
  over HDMI which displays the full 4K resolution when running under
  windows10 but will only reach a maximum resolution of 1920x1080 under
  ubuntu 20.04, same hardware just dual booting. Under Monitor
  Preferences this monitor is detected as an "Ancor Communications inc
  28" and the monitor preferences provides a number of resolution steps
  up to 1920. If I issue "xrandr --addmode HDMI-1 3840x2160" this
  resolution is added to the monitor preferences pick list but
  attempting to apply it results in "the selected configuration for
  displays could not ..." message

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1890418/+subscriptions

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