My oldest ds is 4yr and his pedi was worried about his speech.....He speaks fine (he might say some words a little differently like pasghetti, but not often)....He can also speak some french and spanish but he just doesn't have any interest in having a conversation. My dh isn't worried b/c he said that's how he was until after he started school...He just didn't have anything to say.
Also, it could be b/c he's never away from us so it's not like we can have the "what did you do today?" type conversation. If you ask him a straight-forward question he always answers. He's never really around kids his own age just his little brother (18mo)....they have no problem communicating to each other.