And now I NEED to know if there's some reference I'm missing. I initially thought 4A would be something like DITTO, which would fit the meta clue, but SALLY seems totally random to me. Luckily I got SITBY because otherwise I would've had no idea about 4D (MOM and DAD are also "palindromic relations", the first ones that came to mind)
"4A" as in "foray" as in a definition for "sally" (lower case) as in "a sudden charging attack; a sortie." No reference here, just some playing with number-letter combos!