4HDD8? i think you're reading it wrong.
4HDD is '93/'94 v6 floor shift box. there is no 4hdd8. all ACDelco transmissions have a 1 numeric/3 alpha code.
the way to work it out is this. the number represents the year it was built, in this case 1994
H represents holden, as the box is used by many car builders.
D represents the engine or shifter configuration (D = floor shift v6, C = column shift v6, B = v8)
D represents electronic control (M being hydraulic)
but to answer the question, a 4HDD can be replaced by a 4HDD