Japan domestic is not so strict (int'l may be different, especially if its a non-Japan based LCC), but any weight check is not done at security, so 3 or 4 kg could easily be put in one's pockets, under the armpits, wrapped around the waist like a coat (with things also stuffed in those pockets) or just shoved into your waist.
On multiple occasions around the planet I've carried well over 10kg on my person separate from any carry-on.