4 D is just a 'the whole clue is the definition' - a tack has a big head and it's used to nail down carpets, to make sure they stay down, for security reasons.
6d.
Not sure what to make of this clue!
"Roughly" could be "c" the abbreviation for "circa",
speak could be "utter".
A hewer could be a cutter.
But, sorry, Whetstone, I can't nail the parse!
The ‘bluff king’ is HAL (Henry V) + circle O gives HALO. I think the remainder is suggesting that he should not warrant a halo as he was a bit of a scoundrel :-)