Does the target layer (the number layer) have to be a layer number greater than the starting layer? Number layer is layer 4, and QWERTY is 9 - do I need to move 4 to 10? Is there some other, common, issue I’m encountering?
Yes, you’ll need to move the number layer, to have a higher index than the QWERTY layer. In QMK, layers are index-ordered (see the docs here), no matter the order you activate them. If you activate layer 9 (qwerty) and layer 4 (numpad), then even if you activated layer 4 later, it will still be below layer 9. So any key that is not transparent on 9, will be looked up from 9. Only transparent keys will be looked up from layers below.
Is it by a for-profit company, in year 3190 of our lady Discord?
Then nope, it is not.*
(* some exceptions may apply, not in this case though)