keithl... I'm not online at work and don't keep my Pans Ops book at home. Will have to check first thing Monday morning and let you know on Monday afternoon. Sorry about that.
Confess that I haven't needed to look at that aspect for a really long time. Last time I had need of it was when I designed an 8 DME arc for the NDB/DME approach at Mount Hagen (AYMH). That would've been about 1988 or so!
God I'm old!!!
I just hope that it's still in there somewhere. With all the latest changes and reorganisations of the book, it might've been omitted.
I probably should've checked before talking about the 7 NM limit but, like I say, I don't keep Pans Ops at home.
In any event, I think that you're better off basing your argument on the "flyability" of a procedure that incorporates a 4 DME arc. Don't think I'd fancy my chances with it!
I'm guessing that your problem relates to some sort of obstacle-limited departure procedure? If so, you've got the additional argument of a 15 degree bank angle limit, which might make it real difficult to turn onto the arc and then maintain it without a massive speed restriction.