The $4 billion emergency budget sought by the White House is on top of the $8 billion that the Missile Defense Agency has already been granted for this fiscal year, as well as what other military services and agencies are putting into missile defense. Another $440 million was moved from existing programs to antimissile work two months ago, as the North Korea threat became more serious.

In the emergency request to Congress, and in documents made public by its committees, the precise use of the funds is cloaked in deliberately vague language.

Hundreds of millions of dollars, for example, are allotted for what the documents called “disruption/defeat” efforts. Several officials confirmed that the “disruption” efforts include another, more sophisticated attempt at the kind of cyber and electronic strikes that President Barack Obama ordered in 2014 when he intensified his efforts to cripple North Korea’s missile testing.

Using cyberweapons to disrupt launches is a radical innovation in missile defense in the past three decades. But in the case of North Korea, it is also the most difficult. It requires getting into the missile manufacturing, launch control and guidance systems of a country that makes very limited use of the internet and has few connections to the outside world — most of them through China, and to a lesser degree Russia.

In the operation that began in 2014, a range of cyber and electronic-interference operations were used against the North’s Musudan intermediate-range missiles, in an effort to slow its testing. But that secret effort had mixed results.

The failure rate for the Musudan missile soared to 88 percent, but it was never clear how much of that was due to the cyberattacks and how much to sabotage of the North’s supply chain and its own manufacturing errors. Then Kim Jong-un, the country’s president, ordered a change in design, and the test-launches have been far more successful.