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: 'three giraffes in a field with an egyptian theme in the background'. Reverse all words of length '4' in the given sentence.

three giraffes in a field htiw an egyptian theme in the background