When using native resolution (3840x2160) and 100% sampling the only selection I for texture quality is "Low". The only chance to select a higher setting (that is then also used) is to reduce resolution to 1920x1080 and using 50% sampling. When switching back to native resolution and 100% sampling the config dialog still shows the texture setting as high but when playing it obviously uses low-res textures and when returning to the configuration it also shows low as selection.
Manually editing the config file Arma3.cfg using the settings from Windows didn't show any effect.