No I'm pretty sure it's law, as the WAHR IS law, and SG is about how you work within the law. I've heard of cases where scaffs have fell then been prosecuted while they recover in hospital because they didn't adhere to SG rules (infact the SG booklet mentions this)
TG is purely guidelines, and as far as I know you can only be prosecuted if it all comes crashing down.....
SG4 is just NASC guidance, which obviously shows ways of confoming to the WAH regs.
SG4 is not law but the WAH regs most certainly is. So if you can find a better way of scaffolding other than an option given in SG4:10 that conforms to the WAH regs, then crack on - you're doing nothing wrong....