Which of the following statements(s) is/are correct?

1. If a linearly elastic structure is subjected to a set of loads, the partial derivative of the total strain energy with respect to the deflection at any point is equal to the load applied at that point.
2. If a linearly elastic structure is subjected to a set of loads, the partial derivative of the total strain energy with respect to the load at any point is equal to the deflection at that point.
3. If a structure is acted upon by two forces system $P_{a}$ and $P_{b},$ in equilibrium separately, the external virtual work done by a system of forces $P_{b}$ during the deformations caused by another system of forces $P_{a}$ is equal to the external virtual work done by the $P_{a}$ system during the deformation caused by the $P_{b}$ system.
4. The shear force in a conjugate beam loaded by the $\text{M/EI}$ diagram of the real beam is equal to the corresponding deflection of the real beam.