Here are the completed sentences:

1. New York is the largest city in the United States.
→ No other city is larger than New York.

2. I have never visited such a crowded city as Tokyo.
→ Tokyo is a crowded city.

3. They couldn't turn up on time because of the traffic jams.
→ The traffic jams prevented them from turning up on time.

4. I’ll be happy to give you a lift to school.
→ A lift to school is what I’ll be happy to give you.