Ditto to the others ....
4P ... I too, would probably complete regularly, then put it on the PP being careful to get it smack-dab-in-the-right-place!
Y-seams ... yup, can be done! Fiddly as all get out, but totally rewarding when you finish.
The other option is to cut the triangle of the Y in half, to give you a straight line seam through the background. KWIM? In the end, the same look, but no Y-seam to do. But the background now has that seam instead of being totally plain.
Curves ... no you did not ask about them, but being that some folks will tell you you can't do them in PPing (as with y-seams) just wanted to reassure you for when the time comes, that yes-you-can!!!! Been there, done that. Yes again, fiddly ....... but was ever so happy with the results! Obviously, I don't listen well when people tell me I can't do something!!! And I have a feeling that you are much the same MarionQuilts!!