My son was like that to a large extent. He would actually participate in group activities and would speak when spoken to, but during free-play, he literally huddled in the "train corner" and just drove those little wooden trains around the little tracks, around and around, until I picked him up.
The three things I took away from this were:
1. Some kids just need an extra year before they start preschool. My son would've done great starting at 3 1/2, though that wouldn't have worked with where his birthday falls.
2. Paradoxically, more times per week are better. At this age, kids really have no concept of weeks. To them it's like, they're normally at home, in their comfort zone, and then suddenly, unpredictably, they're plunked into this alien landscape. If it's something they do every day, it's easier for them to develop a productive routine.
3. Most importantly, not all preschools are created equal. My son first went to a "play-based" preschool. It sounded wonderful to me. Who can argue with play? Wrong, wrong, wrong. It was utter chaos, unbelievably loud, all day long. When we switched him to a Montessori school, which was very calm, quiet, and structured, he absolutely thrived.