One of the main properties that your circuit design must have is Modularity that is you build a single module and then combine this module with other modules to build a bigger module, this help making your design easier.
Now back to your problem; In the first picture you provided the implementation of a 4:1 multiplexer
which could be described by this black box
Now the behavioral description of this black box is
Now in order to create the 4-bit version of this multiplexer which could be descried with this black box
The behavioral description of this black box is
output[3..0]=d0[3..0] //the 4-bit output is equal to the 4-bit input
This new circuit has 18 inputs [2 for selecting and 4-bit 4 inputs] and 4 outputs
The implementation of the new circuit would be
However in your design
- You are using an 8-input mux instead of the implemented 6-input mux ! this is not the same module you already implemented as a 4:1 mux !
- You are connecting the select bits of all the multiplexers to another decoder this will always make only one of the select bits 1 and the other select bits are 0 [since they are connected to a decoder] however this violates the behavioral description of our 4:1 mux
So in order to correct your design
- remove the decoder
- use the already implemented 4:1 mux with the correct number of inputs
- connect all the select lines s0 and s1 to the same select lines of your bigger multiplexer