Having Raj be able to talk to Penny will provide for more jokes and storyline opportunities over continuing the 3.5 year joke of him not being able to talk to her unless he's drunk.
Now that we are nearing the end of the season, now's a perfect time for this to happen that can lead into season 5.
Have an episode where Raj starts talking to Penny naturally. Everyone looks confused and asks if he's drunk/been drinking. He says no. Lots of joke opportunities with this alone. Also have some hot girl enter and Raj becomes shy and mute around her. The gang looks more bewildered. Penny then gets bummed out because her looks have faded in a way (not physically of course) and Raj now only sees her as "one of the guys." Her looks have no control of him anymore.
BUT as the hot girl entering shows, he is still shy and mute around hot girls, so that quality doesn't go away; just now he is confortable with Penny and doesn't see her as that hot girl, but as a friend/group member. Best of both worlds; creating Penny/Raj interaction finally but still have him mute around hot girls he doesn't know or not know well.
That makes it more realistic too, since he doesn't lose that shyness/mute quality around hot girls. It's just now it's been a few years he doesn't see Penny that way anymore.