Electric Eel is supposed to allow RaidZ expansion, where you can add additional disks to a RaidZ.
If you want to migrate from a 4 wide raidz2 to a wider raidz2 right now, you essentially have to replicate your data from one pool to another. Today.
Now, there are tricky things you can do to get from pool a to pool b.
For example, if you purchased another 2 drives with the goal of getting from pool a to pool b with 6 drives, instead of 4, but still just two drives of redundancy, you could remove 2 drives from your current pool (leaving no redundancy) and make a 6-way degraded raidz2 with the 4 drives (2 new drives, 2 removed drives), again, no redundancy, which means your data is at risk unless you have a backup, and then replicate across. then replace the degraded drives with real ones. This risks your data, but it illustrates the shenanigans that can be done.
A safer approach would be to take 1 drive from your original pool. Then with the 3 “new” drives, partition each 2 ways… then build a raidz2 with the 6 partitions of the 3 “new” drives. Now either of your pools can withstand 1 drive failure… and you still have enough storage space to replicate everything.
Then you replicate everything.
Then you replace each partition device in your new pool with a drive from the old pool, and when you replace a pair of partitions you can then reuse that drive to replace the rest.
Eventually you have a 6 way raidz2 made out of actual drives instead of partitions, and the raidz2 will resize.
The above requires a lot of command line hackery but the partition to disk replacements can be done in the gui. But if you can’t figure out how to do it via searching this and the old forum, you probably shouldn’t do it.
Or alternatively, you plan ahead and use 6 or 8 disks now 