Those Burndy-type flag terminals are commonly used by the utility guys and a little less so industrially and commercially. They take a slightly different set of dies than more common compression terminals. Whenever I've used them I've had the right tooling but admittedly this is
exceedingly rare, it's usually the RE (Real Electrician) and not the EE (Electrical Engineer) doing the work that would require them.
Never tried one in the set of tri-form FTZ crimpers (designed for the MagnaLug type crimp) I have personally. Might work, might not. No idea.
Using copper tubing and pipe for electrical terminals isn't as strange as it sounds. A tube is basically how some types of them look before being stamped into shape. Tyco and Molex would draw the tube to the exact diameters they need but if you use a wire gauge that happens to match the tube or pipe there's no reason it couldn't work. There's many ways lugs are made. Some are flat blanks where the cable end is rolled, some are cast. Look at the flat end, sometimes you'll see it's obviously stamped flat, like this Panduit tubular lug.
The thing to calculate is cross section of pipe/tube. You need enough material to do the job. A 4 AWG has a spec OD of 0.2043 inches and 41,738 cmil of cross section, which is roughly 0.033 sq-in.
Soft copper 1/4" ACR with OD of 0.25" and ID of 0.190" would have 0.022 sq-in cross section of copper, so technically the amount of material in the flattened tab would be less than the wire, around the equivalent of between 5 and 6 AWG wire. Not ideal but honestly likely no worse than cheap hardware store terminals, some of them do indeed have very thin walls that I find difficult to crimp well. The dies are sized based on the assumption of wall thickness and when it's exceptionally thin you may have to use the next smaller size and stop the tool before it fully closes.
Some dimensions for hard copper plumbing pipe.
Also for the soft refrigeration copper tubing you get in coils.
