The four devices concern different functions within discourse. A single work can exhibit several of them, but their purposes are not identical.
DeviceMain function
Husn al-ibtida’Begin with fitting, effective language
Husn al-takhallusMove coherently from preface to main topic
Bara‘at al-talabCommunicate a request through a skillful hint
Husn al-intiha’End with fitting, complete expression