4034 Vishnu (provisional designation: 1986 PA) is an Apollo asteroid, 0.4 kilometers in diameter. It completes one revolution around the Sun almost once every year. It was discovered by Eleanor F. Helin at the Palomar Observatory in San Diego County, California on August 2, 1986. It is an O-type asteroid, meaning it is similar to the asteroid 3628 Boznemcová, which is the best asteroid match to the spectra of L6 and LL6 ordinary chondrite meteorites. L and LL chondrites have lower iron and metal content, but higher iron oxide content in the silicates.
Its highly eccentric orbit crosses the orbits of Venus, Earth, and Mars. From 1800 to 2200 the asteroid passes within 30 Gm of Venus 71 times, Earth 29 times, and Mars 7 times.
It has been observed by radar astronomy and the orbital solution includes non-gravitational forces.