Hello!
So, using her adjusted age of 4w, her A time would be ~1h to 1h15. This is just a guide--from the link I gave you on the shh/pat thread, so perhaps she would like a little more or a little less. Probably at her age, she would do less if anything. Not all A times have to be the same length. Here's a wonderful explanation of A times:
http://babywhispererforums.com/index.php?topic=128419.0Now for her yawning right after eating, I would say to wait a few minutes to see if she perks up or shows sleep cues still. I made the mistake when DS was very little of following the 3-yawn rule too soon. Once I learned what A time he should be around, it taught me that when he yawns at 35m after being awake, he was probably just yawning; but if it was at 50m, he was probably ready for his nap and I'd start the wind down routine.
S time starts when she's asleep.
As you're trying to figure out her sleep window, just keep in mind that if she wakes up early from a nap and you're not able to resettle her within 15 minutes or so, go ahead and get her up, feed her, and then start the wind down routine again. LOs aren't able to handle a full A time on a short nap, so she'll probably be ready to go to sleep after about 45-50m.
Hope that helps a bit!! Let me know how it goes