"Solar physics is the branch of astrophysics that specializes in the study of the Sun."
The study of the Sun, including its plasma physics, solar flares, and space weather phenomena.
"It deals with detailed measurements that are possible only for our closest star."
"It intersects with many disciplines of pure physics, astrophysics, and computer science."
"... fluid dynamics, plasma physics including magnetohydrodynamics, seismology, particle physics, atomic physics, nuclear physics, stellar evolution, space physics, spectroscopy, radiative transfer, applied optics, signal processing, computer vision, computational physics, stellar physics, and solar astronomy."
"Because the Sun is uniquely situated for close-range observing (other stars cannot be resolved with anything like the spatial or temporal resolution that the Sun can), there is a split between the related discipline of observational astrophysics (of distant stars) and observational solar physics."
"The study of solar physics is also important as it provides a 'physical laboratory' for the study of plasma physics." Unfortunately, the paragraph does not provide specific quotes for questions 2, 3, or 4. However, the paragraph as a whole answers these questions.