If that's saying a 9.5x18 ET12 will be ok on the rear, then a 10x18 ET12 for examples sake will be a further 6.35mm on the inside AND outside (ie, stick further in by 6.35mm and out by the same amount).
So thinking of inner clearance now, a 10x18 ET20 would move the wheel a further 8mm inbound of where the 10x18 ET12 would be, so a total of 14.35mm further inbound from where the 9.5x18 ET12 would have been.
The outer edge of the wheel would sit just 1mm (and a bit) further towards the hub than the TE's in 9.5x18.
So bottom line, if the 9.5x18 ET12 TE's fit ok on the rear, and there's at least 14.35mm of clearance to any suspension components (plus a few extra mm to allow suspension travel and movement of anything around there), then they'll be ok.
That part I can't help with too much cos I don't have a 4 to look at:(
No good asking Japan either cos they only ever list silly offsets that means the wheel doesn't sit flush with the arch, but lost under it instead:innocent: