luci-mod-network: enforce IPv4 mode for traceroute call

Recent busybox traceroute applet versions support both IPv4 and IPv6 and
default to IPv6 when running a route trace to an IPv6 enabled target
host. Pass the `-4` flag to enforce IPv4 mode in this case.

Fixes: #5155
Signed-off-by: Jo-Philipp Wich <jo@mein.io>