[obaslg]

A possible explanation for a perceived performance drop in a 4-man:

I assume that the starter's last inning of each start is worse than his others - he's tiring, and obviously his performance that start is dropping, or he wouldn't be pulled in the first place. In a 4-man, the SP will have more starts, but fewer innings per start, so that the one bad inning (his last inning) per start makes up a larger portion of his total start, so his ERA is worse.

E.g., if your SP would average 7 IP per start after four days off, but only 6 IP per start after three days off, in a 5-man rotation about 1/7 of his IP are these "last innings," while in a 4-man 1/6 of his IP will be "last innings."

However, this shouldn't make a difference to the team ERA. Either way, unless there's a complete game, there will be one of these last innings each game regardless of rotation size.

I want to emphasize again that I haven't seen any performance drop in th3 4-man, and I wouldn't trust it either way w/o a study. If you've ever run the same season ov3r a few times, even with injuries turned off, you've seen the huge range of results possible just by chance.