how does the opening of chapter 5 contrast with the end of the previous ?

The battle in chapter 4 holds nothing but carnage. Limbs are blown off; men with burnt lungs (from the gas) lay strewn about the trenches and battlefield. Kat and Paul attempt to help a boy with his hip blown off. Contrast this to the beginning of chapter 5 where Paul and his comrades are de-licing but enjoying each others company. The scene is more peaceful as they discuss what they might do when the horrors of war (like Ch 4) are finished.