No offroading? I kinda said the same thing with my YJ until I found myself in an area with a lot of decent OHV trails (So Cal.). OHV trails don't cost anything to use unless you break something.
I'd personally go TJ 6, 5spd for on-road only unless you are "MacGyver-Hardcore," a bit of a masochist and want to give the others, the TJ4, or YJ4, auto/5spds a shot. YJ6 is not going to have the coil suspension of the TJ6, so some ride quality and articulation will be sacrificed
Frankly, though, The Great Equalizer for Wranglers is off-road driving -- 4 or 6, YJ or TJ, if properly set up, they are all going to do fairly well off-road. 6cyls will have more torque for sure, but stock low range gearing almost triples torque getting to the ground with a 4 cylinder, so you'l be getting about 300lbs/ft
at the wheels with the 4 off-road in low range which is pretty darn good. Course, in this day and age, the 4 is nice, because it's good off-road and it doesn't kick you in the head at the pump for buying a 12mpg 6 cylinder
My YJ4 5spd is going on 13 years old and is great off-road on difficult Cali OHV trails with just a $250 Aussie auto-locker in the front end.