This assumes you have used /dev/sdb1 vs /dev/sdb and you want to grow /dev/sdb1.
Fire up a LiveISO and GParted for the GUI, but really you'll need the CLI for this.
Use GParted to delete/clear space after ZFS partition.
Then use parted to resize ZFS partitions like so:
parted /dev/sdb print resizepart 1 4000GB (4TB) print quit
Refresh GParted to confirm!
Before closing LiveOS I would consider disable ZFS automount in prod system, boot, configure, test and then re-enable, reboot confirm.
To Grow from real OS once booted back:
zpool list zpool online -e poolname /dev/ zpool list