2nd up stairs toilet

I would like to add a 2nd toilet on the second floor.
The existing toilet is approximately 36" away on the opposite side of the wall.
The garage is directly below w/ floor joists are 16" manufactured type.
Can I plumb directly into the existing 4" supply pipe?

toilet

Yes, but first you have to do something to provide a vent for the first floor bathroom, since you will be using its vent for a drain. There are other "theoretical" steps to take, but in your case you will probably get by without them.

second toilet

Are you saying that you want to add a second toilet on the second floor along with the first toilet that is on the second floor which is on the other side of the wall? If so yes you can cut a 4x3 y into you waste line and plumb 3" to this new toilet. It technically should be vented but it will work fine with out the vent.