You slightly off there, gir. The majority of book 4 takes place at the same time as the first 3/4 of book 5. There is no overlap between books 3 and 4, but tons of overlap between books 4 and 5. Book 4 is quite slow. It deals with a lot of areas that simply didn't get as much attention in the previous novels. It also does…
as Uly said: 2 planes (quads) intersecting each other, so it forms a + if you look at it from above, (= 2x2 = 4 tris) then 2 quads for the axe so you can mirror that. (4 + 4 = 8 ;))