Micha'el is the Angel of mercy (he is made of water), Gavriel, the Angel of Din (he is made of fire) and Eliyahu, the Angel of Redemption. Hash-m's Midas Rachamim (quality-trait of mercy) works swiftly, whereas His Midas ha'Din (quality-trait) works slowly (because He always gives people a chance to do Teshuvah). The Midas ha'Din that involves taking a person's life works even more slowly (which explains why the Angel of death requires eight flights).

Once we are sent into Galus (exile) the redemption, as we see in practice, requires a lot of merit, since Galus takes a long time to reverse. This is why Eliyahu needs 4 flights.

See Agados Maharsha, on whom my explanation is based, who elaborates this further.

Kol Tuv,

R. Eliezer Chrysler