Partial Mash grain bill question?

I'm on my 5th partial mash brew and I'm looking to do a stout. I have a 4 gallon pot that I figure can handle roughly a 6 lb grain bill. Problem is the recipes I'm looking at all have about 4.5 lbs of specialty grains and I've read that the base grain to specialty grain ratio should stay 1:1. Is this a pretty steady fast rule? What would happen if I were to mash with 4.5 lbs of specialty grains and say 2 pounds of base grains?

As specialty grains cannot convert their own sugars, they depend on the diastatic power of base malts to do so.

Mashing with more specialty than base will leave you with more non-fermentable sugars.