Not sure if the Pi4 can have more than one drive attached at the sametime, but you can actually copy the data over while the node is alive using rsync, and then once you have it copied over, do a few more rsyncs until the amount of data that has changed is tiny, take the node offline and then do one final rsync (with the --delete flag). That is what I do (am doing right now) to move from a 3 TB iSCSI to a 15 TB iSCSI disk.
Here is a thread I created to discuss this very way of doing it.