why is a solar day about 4 minutes longer than a sidereal day?

Answer 1

Because Earth is orbiting the Sun.

The Earth rotates 360 degrees so that a star that was directly overhead is again directly overhead. It does that in 23h 56m 4s. But during that time it has orbited almost 1 degree more around the Sun, so to bring the Sun back to the same place in the sky it has to rotate almost one more degree. Basically, a sidereal day is exactly 360 degrees of rotation but a solar day is almost 361 degrees of rotation.

Another way to think of it is that as Earth orbits the Sun, the Sun is seen slowly drifting against the background of stars. So since the Sun moves with respect to the stars, so if the stars make a fixed reference point to measure the rotation against, the Sun is a moving reference point, and it takes that extra 3m 56s for the Earth's rotation to catch up with that moving reference point.

Answer 2

Correct.
The orbital time for the Earth is 23 hours 56 minutes, but because the Earth is orbiting the Sun, the same point on Earth doesn't point to the sun until 4 minutes later.