Montesquieu, Hobbes, Locke, and Rousseau are all 'state of nature' theorists—they attempt to explain politics and government by constructing a thought experiment of an original state of nature which led people to enter into some kind of tacit social contract.
The primary differences revolve around two axes: their descriptions of the state of nature and the motivations for humans to enter into a social contract with one another.
Hobbes held that lives were "solitary, poor, nasty, brutish, and short" in the state of nature (Leviathan, I.13) and that it was a desire for self-preservation that led us to form a social contract because, in nature, life was a zero-sum game.
The other three, however, were more optimistic about the state of nature. Montesquieu, through his parable of the Troglodytes in the Persian Letters argues that people were originally good and benevolent when they led simple lives but that war and politics corrupted them.
Locke thinks that humans were originally free "to order their actions, and dispose of their possessions and persons, as they think fit, within the bounds of the law of nature" (Second Treatise on Government, section 4). They enter into a social contract for mutual benefit because of conditions of scarcity.
Rousseau argued that we were born neither benevolent nor malevolent but as tabulae rasae that were written on by education and environment. He did think that there was true freedom in the state of nature but that social contracts were needed in order for humans in society to be "forced to be free."