is the Catalytic converter needed ?

First thing that will happen is the CEL light will come on .. thus making it impossible to pass emissions because that code stays in the system even after you reset it ,, most emissions inspection require that any reset done is at least a week old with at least 100 miles since reset ,, then you may find that your gas mileage takes a dive because many cars tune the gas rich/lean on the O2 sensor input voltage and on a failed O2 the engine reverts to rich

Can I get passed emissions with 3 of the four cats on my expedition 97 one has whole can I replace with pipe?