Wanted to add a 2nd floor bathroom. I have a 4" DWV from roof to first floor bath. The whole house is on a slab. Needed to know if I can tie into the 4" pipe for the toilet,sink and shower on the 2nd floor. The 2nd floor toilet is right above the first floor toilet. I here stories that when I flush the 2nd floor toilet that it will cause a vacum in the first floor toilet. Others say it will work fine being a 4" pipe.

Wanted to add a 2nd floor bathroom. I have a 4" DWV from roof to first floor bath. The whole house is on a slab. Needed to know if I can tie into the 4" pipe for the toilet,sink and shower on the 2nd floor. The 2nd floor toilet is right above the first floor toilet. I here stories that when I flush the 2nd floor toilet that it will cause a vacum in the first floor toilet. Others say it will work fine being a 4" pipe.

You should be fine if you vent the lavatory and wet vent the tub/shower drain back to the lavatory drain. The second floor toilet should present no vacume on the first floor bathroom group as it will be well vented by both the main stack vent and the lavatory vent. You may revent the lavatory vent back to the main stack vent instead of running it out the roof if you wish.
Regards, tom