4. Tom can't say who the murderer is because he

The text states, 'The murderer is a violent man, Injun Joe, and Tom and Huck are too afraid to say anything about it.' This indicates that Tom knows who the murderer is but is afraid to reveal it. The options are 'doesn't know', 'is not sure', and 'is afraid'. Therefore, Tom can't say who the murderer is because he is afraid.