Wack??
There are certain circumstances under which all four could log flight time, however, there are no circumtances under which all could log pilot in command time.
If a view limiting device is used for one pilot at the flight station controls, then he or she, if rated in the airplane and acting as sole manipulator, may log PIC for the time acting as sole manipulator. See FAR 61.51(e)(1)(i).
If the other pilot at the controls (assuming a light twin, from your description) is acting as safety pilot, and also as Plot-in-Command, that pilot may log PIC time for the time spent acting as PIC of an aircraft for which which more than one crewmember is required, by the regulation under which the flight is operated. See FAR 61.51(e)(1)(iii) and 91.109(b)(1).
A flight instructor, acting as an authorized flight instructor, may log PIC for all time spent acting as an authorized instructor, as given under 61.51(e)(3). Undoubtably this would provide justification for adding a third pilot, in the "back seat," who is logging PIC time. Circumstances to exist in which a flight instructor conducts instruction from other than a pilot station
FAR 61.1(b)(2)(ii) defines "Authorized Instructor" as a person who holds a current flight instructor certificate issued under Part 61 of this chapter when conducting ground training or flight training in accordancee with the privileges and limitations of his or her flight instructor certificate.
61.195(g) requires that an aircraft used for training have two pilot positions, with one exception, and that exception still requires training in a like aircraft with two pilot stations. This paragraph also requires conformance with FAR 91.109, which makes a similiar provision. Nowhere does it state that the flight instructor must occupy the second flight station, but the implication is clearly spelled out in the title of the paragraph for 61.195(g), which reads, "Position in aircraft and required pilot stations for providing flight training."
The guiding principle is that the flight instructor must determine that the flight may be conducted safely. A number of schools do provide instruction in this manner. However, you can bet that in the event of an accident or incident, both the FAA, the insurance, and the courts are going to look very closely at the rationale behind this arrangement in the cockpit. Adequate justification will have to be made for initially placing the authorized intructor in a position where he or she cannot effect recovery or correction with the flight controls, and in the event of an accident or incident, the flight instructor will come under heavy scrutiny for failure to prevent that occurance from happening.
There is no provision for the fourth pilot to log PIC time.
In the scenario you describe, if the right seat is occupied by a flight instructor (holding a multi engine rating on his or her flight instructor certificate), and that instructor is acting as an authorized instructor, there is no provision to allow the instructor in the back seat to log any of the time as pilot in command, or second in command.
If two pilots are switching back and forth and acting as sole manipulator of the controls during the course of the flight, then they may log pilot in command only for the time spent acting as sole manipulator of the controls.
The one other possible exception is that the instructor in the right seat is acting as an authorized instructor, and as safety pilot. The instructor in the back seat agrees to act as pilot-in-command. In this scenario, the "student" under the view limiting device logs time as PIC as sole manipulator, the instructor in the right seat logs PIC as an authorized instructor, and the instructor in the back seat logs PIC as the acting PIC of an aircraft requiring more than one crew member under 91.109(b)(1).
However, in this scenario, only two crewmembers are required, and the FAA will question the validity of the third pilot acting as PIC. Changing the scenario slightly, if the fourth pilot acts as PIC for the flight, and the second instructor in the back seat is providing instruction on giving instruction, to the instructor in the front seat, the four dingbats in the geronimo have their justifiction, albeit misguided. In this case, one would suggest that the pilot under the view limiting device is logging PIC as sole manipulator, the front seat instructor is logging time as PIC as an authorized instructor, the rear seat instructor is logging PIC an an authorized instructor while providing instruction to the front seat instructor, and the rear seat non-instructor is acting as pilot in command of an aircraft requiring more than one flight crewmember, and therefore also logging PIC.
This doesn't wash.
For the reasons previously mentioned, the validity of a pilot acting as PIC, while riding in the back seat of an aircraft certificated for one pilot, will be questioned. Likewise, the validity of an instructor acting as an authorized instructor but not occupying a flight station (while legitimate in certain cases) will come under scrutiny. Certainly if any of this logged time is to be used toward any certificate, rating, or to meet recency of experience requirements. Further, the validity of one instructor providing instruction to another instructor, who is already an authorized instructor, is questionable.
Note also that no one but the manipulator of the controls may log the landings, or approaches. All pilots who are logging time (under applicable regulations) may log the conditions of flight, such as night, instrument, etc.
Hope that helps.
