使用mutate函数在Dataframe中创建新变量的问题

To exclude all the tweets, which dont contain at least 4 words, i want to create a new variable via mutate funtion. "mysontweets" contains only the text variable of the dataframe "tweets_son.df" I tried alot, but i doesnt work. Anyone up for some help? Obviously there is a problem with the inverted comma.To exclude all the tweets, which dont contain a