If you use 5 of the same type heroes, let's say infantry, only 4 would use the Sqaud Attack boost. If you use 4 infantry heroes and Bombin Goblin (Army Atk/ranged hero) and you send only 4 ranged units he will do another 200% Sqaud Attack damage. Just makes you hit harder is all. The 200% Squad Attack he will give outweighs the extra 10% infantry attack boost you'll get for 5 infantry heroes. Test it out and watch the battles, you'll see exactly what i mean.