Rabbinic updates to the purity of woman and updated practice of Jewish females. All these updates are to make a guard as to protect from inadvertent transgressions
Many details of the writing, erasing, drinking of the sotah and more of many details and requirements. In perek 3 the focus was rulings regarding the *drinker* in perek 4 the focus is rulings extrinsic of the drinker.
Financial agreements to exclude the husbands rights to the wife's assets when and how. Does the husband receive return of investment in wife's asset upon divorce? Words which cause acquisition due to joy.