Done! In case anyone else needs plugs for their CTS4 3.0L, here's the info for NGK plugs (my preferred brand):
NGK 6509, LTR6IX-11, Iridium IX
Gap: .044"
Socket size: 5/8"
Amazon has a slightly better price than RockAuto.
I just replaced mine a couple of days ago, super easy and recommended if you have no proof it hasn't been done before.
This is what I got (can't find the link for the pack of 4): https://www.amazon.com/gp/product/B00J62NNAC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1