The number 4 comes from this bit in CorePlayerAbilities.xml:
<Description>The amount of traits this faction can support.</Description>
It doesn't need to be in a raceconfig if the number is the same as the defaultvalue, if the tag isn't there it will go to the default value (this apples to all PlayerAbilityType and all UnitStatType as well).
Added thread to https://forums.elementalgame.com/436142
Edit: One more piece of information. If the A_Traits for a faction is lower than the amount of traits for a unit design, the unit design will not show up in the training window. Be very careful about lowering the amount of traits, and the impact this can have on the AI being able to build units.