Sum of products

Number Theory Level 5

Let $$\displaystyle a,b,c,d \in \mathbb{N}$$ where $$\displaystyle c\le a\le b\le d$$ such that

$\begin{cases} a+b=cd\\ c+d = ab \end{cases}$

and let $$\displaystyle P=abcd$$.

Find the sum of all possible $$P$$.

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