If I have 4 100 mil allies and got them sold then had no allies and bought a 400 mil ally would I get more plunder bonused????
Slightly more than what you invested since the ally sale made you money. Plunder bonus is based on money spent on allies. Not stats etc. It can be in many allies or just a single big priced ally at 0/0/0/0.
Also if the 400m ally had the same stats as each of the 100m allies, it would increase your bonus indirectly by decreasing your overall "strength" while maintaining the same number of allies. Though at those small of numbers the difference would be minimal .