What is the correct transfer function of a feedback loop if the summing amplifier has different input resistors?
I can redraw the block diagram as follows:



Where 
$$H_3(s) = \frac{R_3}{R_2}$$ and $$H_4(s) = \frac{R_3}{R_1}$$

From this, $$\frac{V_o}{V_{in}} = \frac{-H_1(s)\times H_4(s)}{1+H_1(s)H_2(s)H_3(s)}$$