I find my bitch's heads change after their first heat cycle but not after whelping. Their coats get better after whelping though!

Most of my bitches heads grow after a heat and more after a litter. You're not imagining it and I doubt it would have happened anyway at 4.

I find the girls develop more with each litter, especially in broadness of her head. I also notice more depth of chest and forechest after a litter.

I've never seen nose pigment darken at that age but be happy your girl overall improved alot with her litter. I hope you got some pretty pups as well.