The progression of the four seals is from lust to conquest (the first seal) to civil war (the second seal) to famine (the third seal) to pestilence and death (the fourth seal). This is the natural progression of man’s inhumanity to man. In other words, God simply allows human sin to come full circle, turn in upon itself, and self-destruct.

– Grant Osborne on Revelation 6 in Revelation (BEC)