I found this (and the subsequent discussion) super confusing, so let me restate your plan, and see if this is what you meant? OP described going from 4x disks,

I found this (and the subsequent discussion) super confusing, so let me restate your plan, and see if this is what you meant? OP described going from 4x disks, add 3x, end up with a 7x raidz2 pool. The steps are 4/4 (steal disk, set up new pool) -> 3/4 + 4/5 (copy and destroy old pool)-> 4/5 (add missing disk) -> 5/5 -> 7/7. Your suggestion is to add 4x disks, so you can do 4/4 (set up new pool) -> 4/4 + 4/5 (copy and destroy old pool) -> 4/5 (add missing disk from old pool) -> 5/5 -> 7/7 + spare (hot or cold?).