Vicky is saying you don't need separate bit taps for each bit. You already named the vector correctly (Salida_alu[3..0]) so just connect it directly. The bigger issue I see is that you'll have multiple drivers directly driving ALU_DER. Should there be some logic to select between Salida_alu[3..0] and Entrada_Func_Alu2[3..0]?
I appreciate your efforts & patience.
just perform the following steps,
let me know if this has helped to you or if you have any different concern.