I happened to stumble on a post from fieryrajang without looking for anything WiFi related (sheer coincidence). I thought of your reply and figured I should ask… would these be the instructions to get wifi working properly on 4.0? It seems fairly straightforward, but I have no experience with it so I might be mistaken.
Obtain kernel-latest-qubes-vm package on a usb and you can install just before rebooting on Qubes Install. Ctrl-Alt-F2, mount USB to /mnt/sysimage/mnt
chroot /mnt/sysimage/
cd /mnt
rpm -ivh --nodeps kernel-latest-qubes-vm
Then Ctrl-Alt-F6 to return to installer and reboot.
This way you can have working WiFi without having to require an Ethernet connection first to download the kernel. Note this would set all appVms to latest kernel, you can change default back to stable, and only change sys-net.