You have installed a new fresh Freebsd to /dev/ad4 and want now to set it up while running the old system so you don't have to much downtime.
Using chroot with the following commands:
mkdir /mnt/newfreebsd mount /dev/ad4s1a /mnt/newfreebsd mount /dev/ad4s1d /mnt/newfreebsd/var mount /dev/ad4s1e /mnt/newfreebsd/usr mount_devfs devfs /mnt/newfreebsd/dev mount_procfs procfs /mnt/newfreebsd/proc chroot /mnt/newfreebsd /bin/csh