Why? Lack of depth.
Four legitimate starters is only a good situation if they are backed up by three additional capable starters. Guys who you can use if somebody does poorly, gets hurt, has a bad year. One in the rotation, one in long relief, one (or even two) more at AAA.
This year is a perfect example. On paper, Arroyo, Harang, Volquez, Cueto looks good. Let's just fill it out with Belisle, or Fogg, or somebody. Doesn't matter, few teams have good fifth starters.
That logic leads to disaster. Why? Harang is 4-15. Arroyo just got below 5.00. Cueto has been a good rookie but doesn't have winning numbers. Only Volquez has really held up his end all year.
And what's in reserve? Belisle. Fogg. An unready Bailey. Not enough.
Planning for four good starters without high level depth doesn't work because the four guys will almost always have a weak link or two in a particular year.
Getting another starter should be a priority for the Reds. Harang, Arroyo, Cueto and Volquez sound good, but to get them all going well together is a real challenge and the team needs depth in this critical area.