Definition: In this task, you need to reverse all words of a given length in the sentence. The number of letters in a word determine its length. For example, the length of the word "apple" is 5.
Input: Sentence: 'a grass area with rocks on it and several zebra grazing in the grass with trees in the background'. Reverse all words of length '4' in the given sentence.

a grass aera htiw rocks on it and several zebra grazing in the grass htiw trees in the background