This GM rules that attempting to grapple is different from maintain the hold
Here's that sentence in the context of the description of the combat maneuver grapple:
As a standard action, you can attempt to grapple a foe, hindering his combat options. If you do not have Improved Grapple, grab, or a similar ability, attempting to grapple a foe provokes an attack of opportunity from the target of your maneuver. Humanoid creatures without two free hands attempting to grapple a foe take a –4 penalty on the combat maneuver roll. If successful, both you and the target gain the grappled condition. If you successfully grapple a creature that is not adjacent to you,….
(Emphasis mine.) Because the sentence both appears alongside other information about starting a grapple and uses the phrase attempting to grapple as if grappling had not yet begun in earnest, this GM rules that a humanoid creature suffers that –4 penalty only initially—that is, when that humanoid tries to start the grapple—and not on any later grapple checks.
However, as late as 2015 this is still a point of disagreement on the Paizo messageboards. Ask your GM, or, if you're trying to avoid that penalty, just play a nonhumanoid like a typical aasimar or tiefling.