Tandem drivetrain--right side front drive
I suspect #4 doesn't exist because it makes the stoker's position hard to get power out of. 

If you have one very long chain on the RHS, with the chain passing over and under the stoker's chainring, then there is minimal tooth engagement on top, and  perhaps none at all underneath.  Certainly not the "toothcount/2" teeth engaged on the front wheel.  This would make it very hard for the stoker to get power down.

A solution might be to use some jackwheels, like jockey wheels but to press the chain down around the stoker's chainring to give more tooth engagement.  But this would add complexity and potentially jam up.

Since you only want one chainring, cheat.  Have two at the rear, and one at the front.  Power the rear mech directly off the larger chainring, and use two smaller equal-sized inner chainrings for the timing chain.  You could even use 1/8" chain for the timing chain, and use 3/32" from the stoker's chainring to the cassette/derailleur.



In hindsight I think my suggested solution is similar to your #2.