the install script creates vboxusers group and add tcuser to this group without any problems:
i guess that the tinycore4.x provides a different base of the usb-systems as tinycore3.x.
f.e. debian provides a udev-rule
/etc/udev/rules.d/
KERNEL=="vboxdrv", NAME="vboxdrv", OWNER="root", GROUP="root", MODE="0600"
SUBSYSTEM=="usb_device", GROUP="vboxusers", MODE="0664"
SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", GROUP="vboxusers", MODE="0664"