Doyen, Ive been a structural welder for over 15 years, and have welded countless wind screens made of square tube of various sizes, and every CWI that did the inspections wanted #G and 4G welding papers. I do have 6GR papers, and the only thing I've been asked for them on was welding on a bridge in California, which was on cross braces made of 6" round pipe. Yes, the joint is a y connection. Look at Table 4.9, and it states that a welder tested on plate in the 3G and 4G positions is qualified to weld box tubes in all positions.