I'm a licensed plumbing contractor in three states. 3 inch is a standard trunk line for any 2 and a half bath house. Bigger isn't better, if you go to a four inch line at a end commode, you stand a good chance of water out running the solids, also as stated above a four inch line won't fit in a four inch wall. If done properly it would be a floor flange going into a 4x3 closet bend into a 3 inch line. Shouldn't turn to 4 inch until outside of house where it would be 3inch schedule 40 changed to 4 inch 3034. He's trying to make an upsale. I can put a sewer camera down any line in the state of Alabama and convince you your plumbing is messed up. If it's not broke don't fix it

Is the plumbing company Banks Quarles in Tuscaloosa? Cause they shady.