Hamlet's main strength in the first four acts is also his great weakness: he thinks before he acts.
In many ways, this is a strength. Much as he loved his father and dislikes Claudius, he doesn't rush in immediately after the ghost's revelation and murder his uncle. He recognizes that that would be a rash, hot-headed act that might kill an innocent man. He realizes that the ghost, being a supernatural spirit, might have come from the dark side and be tempting him to evil. Therefore, he shows an admirable restraint and level-headedness in deciding to test the hypothesis that Claudius killed his uncle before he acts.
However, he also tends to overthink things, as he himself realizes. As Hamlet himself says, recognizing his weakness:
Thus the native hue of resolution
Is sicklied o'er with the pale cast of thought,
And enterprises of great pith and moment
With this regard their currents turn awry,
And lose the name of action.
Even after he has established through the Mousetrap play that Claudius is...
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