A tetrahedron $ABCD$ is given such that $AD = BC = a; AC = BD = b; AB\cdot CD = c^2$. Let $f(P) = AP + BP + CP + DP$, where $P$ is an arbitrary point in space. Compute the least value of $f(P).$
To find the least value of \( f(P) = AP + BP + CP + DP \) for a point \( P \) in space, we can use geometric properties and symmetry of the tetrahedron \(ABCD\).

1. **Identify Midpoints and Perpendicularity:**
   Let \( M \) be the midpoint of \( AB \) and \( N \) be the midpoint of \( CD \). Since \( AD = BC = a \) and \( AC = BD = b \), the line segment \( MN \) is perpendicular to both \( AB \) and \( CD \).

2. **Symmetry and Optimal Point:**
   Due to the symmetry of the tetrahedron, the optimal point \( P \) that minimizes \( f(P) \) should lie on the line segment \( MN \). This is because \( MN \) is the shortest path connecting the midpoints of the opposite edges \( AB \) and \( CD \).

3. **Using the Given Condition:**
   Given \( AB \cdot CD = c^2 \), we need to find the specific point \( P_0 \) on \( MN \) that minimizes the sum of distances from \( P \) to the vertices \( A, B, C, \) and \( D \).

4. **Minimization Condition:**
   In general, if \( |AB| \neq |CD| \), the point \( P_0 \) that minimizes \( f(P) \) lies on \( MN \) such that the ratio of distances from \( P_0 \) to \( M \) and \( N \) is equal to the ratio of the lengths of \( AB \) and \( CD \). However, in this problem, since \( AB \cdot CD = c^2 \), we need to consider the specific configuration of the tetrahedron.

5. **Conclusion:**
   The least value of \( f(P) \) is achieved when \( P \) is at the midpoint of \( MN \), due to the symmetry and equal distribution of distances in the tetrahedron.

The final answer is \(\boxed{f(P) = \text{minimum for the point } P_0 \in [MN]}\).