Multiplexer 4x4 Question

Hi All! I've been very much enjoying the Verilog problems recently, they remind me very much of the game MHRD - in retrospect I would guess the game was based off of systems just like this!

I'm having a bit of confusion with the Multiplexer 4x4 though. I went for the first suggested approach of "ternary operators in an assign statement", but it doesn't appear to be working.

I don't want to replicate any code here to spoil anything for those yet to solve, but it seems like a relatively straightforward line of code so I'm feeling like I might be misunderstanding something fundamental... Does anybody want to take a look at my code and maybe give me a hint regarding what I might be missing? Thanks!