Latest Arma 3 1.70 Mac Port, while running under Metal 2.0 on 4K display, does not allow any higher display resolution than 1920x1080. While Mac desktop and monitor is 3840x2160, the game runs 1920x1080 maximum, thus the grahics look blurry and performance probably hits as well (downscaling - the game actually shows higher FPS with 200% oversampling in the display settings than with 100% sampling at 1920x1080), although in gameplay the difference was negligible.

Setting Arma 3 to 3840x2160 manually by editing the appropriate resolution entries in Arma3.cfg (located at ~/Library/Application Support/com.vpltd.Arma3/GameDocuments/Arma 3/Arma3.cfg):
winWidth
winHeight
winDefWidth
winDefHeight
fullScreenWidth
fullScreenHeight
renderWidth
renderHeight
to 3840 and 2160 respectively,

the game crashes, with cause of crash stated as In one go, tried to upload data stream of size 33177600, bigger than maximal supported size 16777216! Crashing out.

Relevant eon.txt logfile info:

[thread 00000002][47922]: eON_WindowEvent: ResizeBuffers 3840 x 2160
[thread 00000012][E][47923]: In one go, tried to upload data stream of size 33177600, bigger than maximal supported size 16777216! Crashing out.
[thread 00000002][47929]: eON_WindowEvent: SetFullscreenState false 0x0

Further relevant info from rpt log:

Original output filename: Arma3Retail_DX11_x64
Exe timestamp: 0000/00/00 00:00:00

Type: Public
Build: Stable
Version: 1.70.0

Allocator: c:\volumes\3tb\steam\steamapps\common\arma 3\dll\tbb4malloc_bi_x64.dll
PhysMem: 14 GiB, VirtMem : 131072 GiB, AvailPhys : 7.7 GiB, AvailVirt : 131069 GiB, AvailPage : 11 GiB

Detected OS X
Reported VRAM size for limiter:8192 MB
Cannot register unknown string STR_VIV_GETIN
Cannot register unknown string STR_VIV_DROP
Cannot register unknown string STR_VIV_UNLOAD

DX11 - Initializing DX11 engine.
DX11 - Using DXGI adapter 0 (detected in config).
DX11 - Using DXGI adapter 0.

• adapter description : Virtual display at (0 x 0)
• adapter vendor ID : 1
• adapter device ID : 1
• adapter subsys ID : 1
• adapter revision : 1
• dedicated video memory : 0
• dedicated system memory : 0
• shared system memory : 0 InitSound ... InitSound - complete

PhysX3 SDK Init started ...
PhysX3 SDK Init ended.
SimulWeather - Cloud Renderer - noise texture file is not specified!
Loading movesType CfgGesturesMale
Creating action map cache
MovesType CfgGesturesMale load time 365 ms
Loading movesType CfgMovesMaleSdr
Reading cached action map data
MovesType CfgMovesMaleSdr load time 4159 ms

Relevant info from crash dump:

Graphics Info:
NVIDIA GeForce GTX 1080� 256 MiB VRAM (device 10de:1b80 rev 161, com.nvidia.web.GeForceWeb)

[thread ffffffff][I][0]: Log system initialised
[thread ffffffff][I][1]: eON_Core::init() - libICU initialised with result 0.
[thread ffffffff][I][1]: Default allowed number of simultaneously open files in the system is 356.
[thread ffffffff][I][1]: Setting allowed number of simultaneously open files to OPEN_MAX = 10240
[thread 00000001][I][1]: macOS version 10.12.6
[thread 00000001][I][1]: Detected 1 Metal devices
[thread 00000001][I][1]: Device 0: 'NVIDIA GeForce GTX 1080', depth24stencil8 format supported, recommended max working size: 8589934592 bytes ( 8388608 kB, 8192 MB )
[thread 00000001][I][1]: supports MTLFeatureSet_OSX_GPUFamily1_v1 feature set
[thread 00000001][I][1]: supports MTLFeatureSet_OSX_GPUFamily1_v2 feature set
[thread 00000001][I][1]: supports sample count: 1
[thread 00000001][I][1]: supports sample count: 2
[thread 00000001][I][1]: supports sample count: 4
[thread 00000001][I][1]: supports sample count: 8
[thread 00000001][I][1]: Using device 'NVIDIA GeForce GTX 1080'

[thread 00000002][I][80200]: eON_WindowEvent: ResizeBuffers 3840 x 2160
[thread 00000012][E][80216]: In one go, tried to upload data stream of size 33177600, bigger than maximal supported size 16777216! Crashing out.

Also, this buffer overuse keeps growing in the error report all the time, creating new streaming buffer:

[thread 00000013][I][25323]: Max dynamic buffer memory overuse now: 3650192 bytes
[thread 00000012][I][25678]: Created another data streaming buffer, 16777216 bytes in size

The machine is perfectly fine running Arma3 in 4K under BootCamp.

One time and one time only, the game changed to 4K in windowed mode in the video settings menu correctly (though windowed only, thus only 1/4 of the screen was visible due to HiDPI desktop), but crashed again after changing back to fullscreen and back.