If you do that, aren't you spending a 4th level slot + a 1st level slot to do the same damage you'd get from a 4th level slot?I get it that you get extra cha damage... but you could do 6d8+Cha + 3d8+Cha with those same slots in 2 turns... or am I missing something?
I think the point is that you get sorcery points (plural) from cannibalizing the 4th level slot. You spend one of those points to twin a first level slot (I assume twin ing a spell lets you cast two version of the same spell in a single round). You still have other sorcery points that you got from the 4th level slot to use in subsequent turns. So it might not always make sense to do what is described above, but it does give you the flexibility of maximizing your damage output.