Sounds like just piecing (sewing and cutting) inaccuracies. I think it looks pretty good for a fourth quilt! But there's always room for improvement. May I suggest a book by Sally Collins called The Art of Machine Piecing? It has improved my piecing skills tremendously, and I have only adopted a few of her suggestions. I took an all day workshop with her once, and it was the best money I ever spent on a class.