One thing about "Spiderweb" that sets it apart from the first three books is the lack of a time frame. Enright wrote the first three during World War II and the war is at the center of the family's lives and is present in each book; the children are busy presenting a show and working after school to buy war bonds and going on scrap metal drives during the summer holiday. The first three books take place from the later winter and early spring of 1942, through the end of the summer of 1943. But although "Spiderweb" runs from October of 1944 to June of 1945, the war is never even referred to in the book. Even V-E Day in May of 1945 which would have been celebrated all over town, isn't mentioned. Perhaps this is because Enright wrote "Spiderweb" ten years after she wrote the third book and many of her readers hadn't been born during the war; but still, some mention of the events would have given the book a dimension that is present in the first three but lacking in this one.
When I turned the last page of "Spiderweb" after reading it as a child, I was devastated to realize that there would be no more Melendy books. But Enright had the right idea; the next year would have seen Randy herself going off to boarding school and leaving Oliver rattling around the Four Story Mistake by his lonesome. A depressing prospect indeed. Enright knew where to end it.
I would echo the reviewer who says that the Melendy books would make a great TV mini-series, excpet that (having seen what TV did to some other classic children's books) I'd be afraid that they'd try to modernize them and mess them up. While the Gone-Away books could, perhaps, survive (they are far less time-bound), the Melendy books are tied very specifically to a particular time/place, and attempts to update would ruin them.