Welcome to Flyertalk.

I have posted my views about this many times - my kids (twin girls) seemed to love the familiarity of their car seat for long-haul flights. I'm confident that they could have sat in the seat without it, but we typically wanted it at the destination anyway (couple of bad experiences with the rented car seats at the car rental agencies taught us to bring our own).

For most of our flights, we were in F or J, where there was plenty of room for the tray table. The angle of the car seat seemed ok for sleeping, although it wasn't reclined as far as mine was. How far does a coach seat on SQ recline, anyway? From my recollection, the car seat typically offers a greater angle of recline than the coach seat itself.

You mentioned that she is four - by the age of four, mine were out of the five-point harness car seat and using a booster seat in the car. For traveling, we used a compact booster seat we checked with the luggage.

Four is probably old enough to sit without it. Good luck.