Anytime I do hand appliqué, I cut the blocks larger to allow for the inward shrinkage. I then use a lot of Magic Sizing to stiffen them when pressing, and square them up. Because you don't have an equal amount of white space at the edges of all of your blocks, I would suggest that you do some kind of pieced sashing that has a light background. Obviously garden maze sashing comes to mind for floral appliqué, but a twisted ribbon would look good and so would an appliqued sashing. But the light background (similar to your blocks) would help keep the more filled blocks from looking hemmed in by a dark sashing like you had. I would also lean towards a wider sashing than what you had; with only four blocks I think it will set them off better, and it will set the stage for you to consider a wider and possibly more dramatic outer border.
She is also correct about the difference in the unequal amounts of white space. If I chose the blocks again, I would have chosen something other than the Dresden plate for winter. I might have still used the “wreath” of leaves for fall, but scaled it to leave more white space outside the leaves.
The other posts done on this quilt:
New Year's Resolution -- Finish This Quilt
Dresden Plate Block – my winter
Windblown Tulips Quilt Block – my spring
Wild Rose Quilt Block– my summer
Wreath Quilt Block – my fall