How to start separately excited motor with 4 point starter
The field must be energized with the rated field voltage before or simultaneously with energizing the armature. I am assuming that by "4-point starter," you mean a starter that connects a fixed resistance in series with the armature and then shorts out the resistance in 3 additional steps ending with the armature connected directly to the supply with no resistance in series.

Re Added Diagram

The diagram shows the field connected to the DC source through a variable resistor. That would be considered to be a separate source. The starting lever connects the field to the source through the brass arc at the same time it is moved to the "1" terminal. If that arrangement is not suitable for the field, the field could be connected directly to some other source, but there should be some provision to assure that the field is energized when the armature is energized. If the field is not energized when the armature is energized, the motor could run to a high speed if the load is light. That could cause the motor to be damaged.