Answer:
Up to Act 4, Macduff seems to be the only one suspicious of Macbeth in the death of King Duncan. Then, in Act 4 scene iii, Malcolm also seems to be wary of Macbeth's involvement in the deaths of many people who are 'supposedly' obstacles to the throne of Scotland.
Explanation:
William Shakespeare's "Macbeth" revolves around the story of how Macbeth came to be the king of Scotland. The play delves into the themes of loyalty, deceit, greed, and the eventual downfall of Macbeth.
Through the 'prophecy' of the three witches, Macbeth feels himself the rightful king of Scotland, thereby seizing power and killing whoever comes in his way. But with the help of his wife Lady Macbeth, and her careful planning, he was able to steer clear of any suspicion from the other characters. Up till Act 4, no one seems to be suspicious of him except Macduff who is loyal to Malcolm, the son of the deceased king Duncan. And in Act 4, we see Malcolm also becoming more aware of the 'involvement' of Macbeth in the deaths of the other important characters such as Banquo and Lady Macduff and her son.