Viet,
I am struggling to help on this one as my area of expertise is ASME IX but hoping some of the D1.1 gurus (Al, Brent, John W etc) might jump in and help.
Firstly, the joint is 2G, not 4G but that still doesn't help you at all.
Al has mentioned numerous times that welders must comply with specific welder qualification tests nominated in D1.1.
A 6G will not replicate what you are doing as even having a single bevel with the flat preparation on the top will require you to catch (fuse) both internal edges of the joint.
The 6GR set-up has a flat preparation with a smaller ID than the bevelled preparation so this is meant to replicate welding a single bevel onto a flat surface (which is your situation)
However,
The restriction ring is meant to replicate the tight confines of a T, K joint as part of the T,Y,K requirements.
You have no access restrictions on what I would consider to be an easy joint to weld so not sure why you would have to perform a welder qualification with a restriction ring.
C'mon Al - help me out here ?
Cheers,
Shane