For 4.0, the difference depends on whether it is installed with legacy mode or UEFI mode. In the former mode, grub is will be used but not in the latter, so that that file is not exists but there is another one called xen.cfg instead.
[root@dom0 ~]# cat /boot/efi/EFI/qubes/xen.cfg
[global]
default=5.4.98-1.fc25.qubes.x86_64
[5.4.98-1.fc25.qubes.x86_64]
options=loglvl=all dom0_mem=min:1024M dom0_mem=max:4096M iommu=force ucode=scan smt=off
kernel=vmlinuz-5.4.98-1.fc25.qubes.x86_64 root=/dev/mapper/qubes_dom0-root rd.luks.uuid=luks-1dc4d737-6337-4cc3-a29d-edf3720a95cd rd.lvm.lv=qubes_dom0/root rd.lvm.lv=qubes_dom0/swap i915.alpha_support=1 rd.qubes.hide_all_usb plymouth.ignore-serial-consoles
ramdisk=initramfs-5.4.98-1.fc25.qubes.x86_64.img
[root@dom0 ~]#
For your reference.