[Malice]

4 gate robo sentry immortal take third, get twilight for blink and +2, sim city third and other entrances and scout with obs to determine what zerg's doing, high sentry count + more immortals if they're doing stephano roach style (constant production of immortals) and just forcefield properly.
When youre playing a passive build like this one, you need to be scouting gas timings for zerg, to see if they're going to be aggressive or passive and teching, or to see what tech path they're likely to take. You can follow this up by getting colossus, and pushing out with +3 for a strong 3 base timing (keep an obs in their base if you can to scout hive timing, when that hive finishes, push out immediately.)
Or conversely, you can take a fourth base and go for that archon mothership lategame composition.

Here's a tutorial made by Whitera on a pretty good immortal sentry build

http://www.youtube.com/watch?v=DZD0MqEorJE

http://www.youtube.com/watch?v=DZD0MqEorJE