I love the book; but I agree that the Delves assume fairly optimised play for a 5-PC group at the listed level. Running Delve #4, the first two fights were pretty easy, but then a reckless PC (hi Francis!) > took the group into the final battle without a short rest, and the 7th level Elite BBEG nearly TPK'd the ca 4th level party.
I think many of the Delves can be used with a typical 6-PC group at the listed level and stay very challenging, while a 4 PC (or certainly 3 PC!) group could easily be defeated.
I think many of the Delves can be used with a typical 6-PC group at the listed level and stay very challenging, while a 4 PC (or certainly 3 PC!) group could easily be defeated.