Defining Your Game | The Angry DM: D&D Advice with Attitude is a good post on what role-playing is and is not.
RP means imagining a hypothetical situation, projecting yourself into the mind of your character, and deciding on a course of action.
I particularly like how he walks through different scenarios and discusses what makes for weak or strong role-playing. He’s right that role-playing takes a secondary role in 4e combat. A typical 4e combat is a tactical game more related to chess than to imagining yourself as your character.
The weakest RP occurs whenever a player doesn’t have to go into a character’s head to make a decision, usually because the decision is based on reason and logic and there is a correct answer to be found. The strongest RP occurs when a player has to resolve an internal conflict for the character, to decide between conflicting goals and motives and establish priorities that might not have been explored before.
His conclusion that 4e is largely role-playing neutral feels right to me. You can play it with strong role-playing, but there’s nothing in the mechanics to encourage you to do so. I’d say that 4e is slightly role-playing hostile in that it’s easy to have situations where strong role-playing would put you at a mechanical disadvantage, e.g. the elf that focuses on killing the orc instead of assisting the party with taking down the big bad.