During 1973 and 1974, the Mothers included George Duke, Napoleon Murphy Brock, Ruth Underwood, Ian Underwood, Bruce Fowler, Tom Fowler, Walt Fowler, Jean-Luc Ponty, Don Preston, Ralph Humphrey, Chester Thompson, Jeff Simmons, and Sal Marquez. These line-ups are featured on the albums Overnite Sensation, Apostrophe, Roxy & Elsewhere, One Size Fits All, and You Can't Do That On Stage Anymore, Volume Two, as well as portions of Studio Tan, Sleep Dirt, and You Can't Do That On Stage Anymore, Volumes One, Three, Four, and Six. This is many people's favorite era of Zappa's career.
These pages summarize the similarities and differences among the six concert tours of line-up 4 of the Mothers:
| Dates | Line-up | Location |
|---|---|---|
| February-July, 1973 | 4a | North America, Australia |
| August-September, 1973 | 4a | Europe |
| October-December, 1973 | 4b | North America |
| February-March, 1974 | 4b | North America |
| April-May, 1974 | 4c | USA |
| June-December, 1974 | 4d | North America, Europe |