Point of the Secrecy?
I agree with all that's been said. Only the emphasis is (or can be) different. For example:
4/Pents = someone who may keep a secret either because they don't want to get involved or involve others ("Not my business!" or "It's none of their business"), or because it's advantageous to them to keep it (like where the treasure is buried). I can very much see this card for those who are so afraid of poverty that they keep several, secret bank accounts. Often, after they die, the bank accounts can't be found. It's not that they took the secret to the grave per se, it's just that they couldn't bare to tell anyone and have their advantage removed. So they died before they were able to pass on the secret.
Fear, worry, self-interest or just avoiding trouble is more likely the motivation for keeping the secret than, say, honor or love or friendship.
4/Swords = There is certainly a "taking the secret to the grave," and in this instance it's more thoughtful. The 4/swords could indicate someone who sees things in broader terms. A person who might say, "The world isn't ready to know this yet." Or, on a personal level, "It would be better if this person didn't know this."
I would also imagine getting this card for a teenager who spends a lot of time in their room writing up things in diaries. Things they don't want their parents to read or see. This is a private phone conversation, a private e-mail, and it's going to get buried in some secret place because it's better that no one else find out about it.
7/Swords = The important thing about this card is that the person is either sneaky--the secret they're keeping give them a hold over a person ("do this for me or I tell!"), or is having to be sneaky against their will. They hate having to keep this secret but honesty would do them no good. Like if you knew your boss' wife was cheating on him, but also knew he'd never believe you and would likely fire you if you said anything.
7/Swords often indicates keeping a secret but not being happy about it. Feeling dishonest or dishonorable about it.
I suppose that 7/Swords combined with 4/Pents, in a worse case senario, might indicate blackmail.