A manager has a team of four people: Adam, Brian, Carol and Diana. Each one has a different project deadline: 3 days, 4 days, 5 days and 6 days, but not necessarily in that order. Adam's deadline is earlier than Carol's. Brian's deadline is not the longest. Diana's deadline is the shortest. What is Brian's deadline?

Let's logically sort out the deadlines based on constraints:
1. Deadlines available: $3,4,5,6$ days.
2. "Diana's deadline is the shortest" $⟹\text{Diana}=3$ days.
3. Remaining values for Adam, Brian, and Carol are $4,5,6$ days.
4. "Brian's deadline is not the longest" $⟹$ Brian cannot have the 6-day deadline. Hence, Brian must have a deadline of either 4 or 5 days.
5. "Adam's deadline is earlier than Carol's" ($\text{Adam}<\text{Carol}$). If Brian takes the 4-day deadline, Adam can take 5 days and Carol takes 6 days, which perfectly aligns with all conditions.

Therefore, Brian's project deadline is 4 days.