Is Volumio and especially the Touchscreen plugin fully compatible with Raspi4?
On Pi 4 the backlight of HDMI displays might not be switched off when the screensaver is active. But IIRC when I tested it last time it worked: With hdmi_blanking=1 in “boot/userconfig.txt” the HDMI monitor I had connected powered off its backlight shortly after the plugin’s screensaver was activated.