I haven't heard of this swap being done before...
Most of us 2.6'ers were desperate to get rid of the carb and vac lines.

However, having gone 4 to 6, going 6 to 4 will still have a few of the same concerns.

If the motor mounts line up, and are interchangeble between the 6 and the 4, then you will be fortunate! Remember, the output shaft from the 4 will need to allign perfectly with the trans input, or you'll be customising motor-mounts and/or trans mounts.

You'll need all the wiring from the 4 that comes out the passenger side firewall, and the computer that's under the airbox (above the passenger's feet). Wiring diagrams for both 6 and 4 will help.

You'll need the device boxes that line the 2.6's inner fender.

The radiators may not swap easily, and will need to if the coolant in/out hoses don't match.

Exhaust may not match up.

In the dash gauge console, the tach will need to be changed (or ignored...), and the fuel tank gauges are different and will need to be swapped, and even though both 4 and 6 gauge consoles use that same round connector, there are some wiring differences, including the high beam indicator. You will need the wiring diagrams for that one.

The air filters are in different places, and the 2.6 needs its filter on top. So the 6's remote filter can probably be left as an empty can. That way at least the air hose connects may be easier to do. Not a big issue though.

...off the top of my head.