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 skateboader is skateboarding down the sidewalkwith his hat backwards'. Reverse all words of length '4' in the given sentence.

a skateboader is skateboarding nwod the sidewalkwith his hat backwards