10-10:30 PM · FX · Returns Sept. 18FX's foulmouthed comedy delves even deeper into the toilet for its fourth season. Literally. ''Frank [Danny DeVito] and Charlie [Charlie Day] find a turd in their bed,'' explains star/exec producer Rob McElhenney, ''and Dennis [Glenn Howerton] and I go into full CSI mode to figure out who did it.'' Also on tap: Sinbad and pop star Rob Thomas guest on a rehab-themed show, and the series travels back in time to reveal how the crew cracked Philly's Liberty Bell. Still, says director Fred Savage, ''the best moments are when it's just the gang in the bar.''