Your question can't be answered without some more info about the design of the plant you are considering. I'm not sure what kind of plant design you are thinking about, that would have four emergency diesels. I'm familiar with the plants in the US, these all (?) have two "trains" of engineered safeguards and therefore two emergency power trains and two diesel generators. Your plant, with 3 or 4 diesels wouldn't make sense unless there are 3 or 4 trains of safeguards (safety injection pumps, contanment sprays, vital HVAC, etc...). And depending on how many trains of ESF you have, that's how many diesels you need.
To reiterate, my understanding of what you mean by "station blackout diesel" is another power source, installed for postulated failures of all of the emergency busses. Since AFAIK, no one postulates an Loss of Coolant accident coincident with these failures, the SBO diesel only needs to power the normal RCS makeup and some HVAC loads (with the emergency feedwater powered by steam driven turbine(s), independent of any AC power). So, the SBO diesel is much smaller than the EDGs.
If the plant you are considering is based on a different design, then you can likely be guided by two ideas: (1) the EDGs each power a single train of safety features, and the number of trains required depends on the single failure criterion plus consideration of allowed equipment outages, and (2) the SBO is a "beyond design basis" event that postulates multiple failures in the AC system but does not assume a coincident accident.
Does that help, or is it more confusing?