does anyone have any idea what this might be on the walls in this pictures I'd like to something similar, but I can't figure out what the material is.
Here is a link that might be useful:
I don't know what the material is but the walls look to be
covered in a ship lap type siding with a decorative edge
between each joint. It may be some type of man made material
since it is in a bathroom and subject to high humidity.
This is all a guess based on a picture.
Do you think I could use SYP 1x4 S4S for this sort of look?
You could. The results would depend on the grade of lumber
and the environment in which it will be used.
Number 2 grade would be as low as I would go and I would
not use SYP in a wet or humid area to avoid rot and/or mildew.