You should write about one poem , then write about the second (comparing to poem one) , then write about the third poem (comparing to 1 and 2) and then write about the last (comparing to the other three). As long as you are comparing all four poems and answering the question it doesn't matter how you do it !!!
Choose the two poems that you are best at and have a similar theme to go into full detail on. You should compare as many features as possible like structure colour imagery etc. and then go into less detail with the other two by just dropping them in to compare to your main poems. Make sure you structure your essay properly (: