Go test it again, because I've tested it and she doesn't end up getting BT PO K auto-throw because she appears in front. Anyway..
Yes, 4A+B6 PO is -3. In fact, it's her "safest" transition into PO. PO K is 10 frames (-21 on block) and her fastest move from PO. And still after blocking 4A+B6 PO, you can use an i14 move and stab her out of PO K for some reason.. so there's something iffy with the frame data/hitboxes. You're able to beat out her fastest option from PO, while she's at -3. BUT OH NO.. 1/2 METER MEANS SHE CAN STOP COUNTERS.. right?
4A+B, B is double bombs from SCIV, doesn't exist anymore. I'll assume you're talking about 4A+B, A. Natsu doesn't get free airthrow like in SCIII anymore, but this can hit you if you try to jump 4A+B. If you do 7_8_9A_B_K (TJ attack), you'll be able to hit her.. depending on how fast she cancels into the A hit. I would just block bombs in general and deal with her PO game. (Can't block bombs, free airthrows all day).
AA beats out BT PO K. I would sidestep it and launch natsu instead though.
AA does not punish 66B, but stab does, pushback or not.
I know what you mean though, other characters have it harder because they lack i14 moves that have range, to deal with Natsu's pushback. The point of this thread was to show how punishable natsu can be, and I've used Pyrrha as a primary example because she had stab, and could deal with her pushback. Other characters will have to rely on other things to punish/restrict Natsu's moveset. Hopefully I'll be able to put something together that other characters can use to punish Natsu (if they can) once I have free time, but for now.. this is all I got.
-ASZ