If I have states $\psi_{a,b,c,d}$ ,then is the following relation true: $$\langle \psi_a \otimes \psi_b| \psi_c \otimes \psi_d\rangle = \langle \psi_a |\psi_c \rangle \cdot \langle \psi_a |\psi_d \rangle \cdot \langle \psi_b |\psi_c \rangle \cdot \langle \psi_b |\psi_d \rangle?$$
I believe that it is true when the states are orthonormal; e.g. with momentum states we have: $$\langle p_a p_b| p_c p_d \rangle = \langle p_a |p_c \rangle \cdot \langle p_a |p_d \rangle \cdot \langle p_b |p_c \rangle \cdot \langle p_b |p_d \rangle = \delta(p_a - p_c)\delta(p_a - p_d)\delta(p_b - p_c)\delta(p_b - p_d) $$
The context for this question is evaluating a scattering amplitude. The probability for an initial state $|\phi_A \phi_B \rangle$ to scatter and become a final state of $n$ particles whose momenta lie in a small region $d^3 p_n ... d^3 p_n$ is given by:
$$P(AB \to 1, 2, ... n) = \left( \Pi_f \frac{d^3 p_f}{(2\pi)^3} \frac{1}{2E_f}\right) |\langle p_1 ... p_n | \phi_A \phi_B \rangle |^2$$
How are we to treat the term $$\langle p_1 ... p_n | \phi_A \phi_B \rangle ?$$