I know this isn't a Windows fix, but I had the same issue under Linux (Fedora) and finally fixed this with the following code:
# UDEV Rules for Arduino UNO R4 boards
#
# This will allow reflashing with DFU-util without using sudo
#
# This file must be placed in:
#
# /etc/udev/rules.d
#
# After this file is installed, physically unplug and reconnect the device.
#
# Arduino UNO R4
# --------------
#
SUBSYSTEMS=="usb", ATTRS{idVendor}=="2341", ATTRS{idProduct}=="0069", GROUP="plugdev", MODE="0666"
SUBSYSTEMS=="usb", ATTRS{idVendor}=="2341", ATTRS{idProduct}=="0369", GROUP="plugdev", MODE="0666"
#
# If you share your linux system with other users, or just don't like the
# idea of write permission for everybody, you can replace MODE:="0666" with
# OWNER:="yourusername" to create the device owned by you, or with
# GROUP:="somegroupname" and mange access using standard unix groups.
#
placed in the file "/etc/udev/rules.d/99-arduino-uno-r4.rules".
This is far from only my work, credit should go to Julien Vanier and a a whole bunch of people who solved this same problem for 'Particle' boards some time ago.
Jeff.