The interior diameter of pex is actually a fraction smaller than copper this forces water to come out of the shower at the same time as the tub spout.
Can i use pex for tub spout.
Copper keeps the spout in place.
You cannot use pex because the pipe coming out of the valve needs to be able to support the weight of the tub spout and handle the abuse of pulling pushing down of the divirter built into the spout.
Long story short it is unacceptable.
After investigating the 3 4 will work just fine no restrictions.
Pex when you pull up on the diverter the entire spout moves.
These spouts will work with just a clean open copper pipe as it has a set screw adapter to clamp the pipe.
2 reasons i learned.
The tub spout can be copper brass or galvanized pipe.
Soldering to the nipple with the pex attached is not optimum but can be managed if you have room to tightly wrap a water saturated cotton rag around the copper and extended over the pex portion.
Pex to tub spout.
It is only created on tub spouts where a diverter closes off the easier path out the tub spout to send it to the shower.
Copper is the only professional installation material.
Pax has a smaller inside diameter than copper when you will turn on the spout your showerhead will also be dripping if you read the manufacture instructions it will say do not use pex for spout level 2.
2nd reason pex is flexible and the spout relies on the pipe for support.
When pex is used on those tub spouts the excessive back pressure sends water up the shower.