You seem to be asking for a distance above the event horizon at which time dilation due to the gravitational acceleration of the black hole would convert 4 billion years to 1 second. As all time is relative, what you really are asking for is the distance above the event horizon at which it would APPEAR to someone on Earth that an event which takes 4 billion years to transpire on Earth, occurs in 1 second.
The Schwarzschild radius marks the event horizon around a black hole. It's the radius of a sphere into which enough mass is compressed, such that escape velocity would be the speed of light. Any object with radius smaller than its Schwarzchild radius (based on the amount of mass in the object) is a black hole. The Schwarzschild radius of our black hole at galactic center is about 13.3 million kilometers. The Earth is about 25 to 28 thousand lightyears above the event horizon of that black hole.
Time dilution due to the gravity of the black hole can be approximated as follows:
Tr/T = (1-Rs/r)^1/2,
"Tr" is elapsed time of an observer at radial coordinate r above the event horizon (1 second),
"T" is elapsed time for an observer on Earth (4 billion years),
"Rs" is the Schwarzschild radius of the black hole at galactic center (13.3 kilometers),
"r" is the radial coordinate for which we are solving.
Solve for "r" in the time dilution equation, and you have the answer. It will be very, very small, and not very accurate!
See ACuriousJim's comment below. He found "r" to be 8.3 x 10^-25 meters. However, the equation above is for an observer hovering above the event horizon, rather than for one who orbits the black hole, as John Rennie points out in his comment, below. Also, it would be impossible for an observer to achieve a stable orbit so close to the event horizon, as Hypnosifl says in his comment, below.