4-way plug includes:
1. ground
2. tail/running lights
3. Left turn and stop
4. Right turn and stop
{The stop light is when both the left and right turn lights are on at the same time - and not flashing. The emergency lights (hazzard warning lights) also use both the L and R turn lights, but with the flasher.}
7-way adds:
5. Electric trailer brakes
6. 12-volt hot (battery charge/maintain)
7. Backup light(s) or auxiliary
So in addition to the 4 wires you have, you need to add
1: one wire through a relay and the truck's brake-light wiring for the trailer brakes. (If the truck's brake light comes on, the trailer brakes should be engaging too.)
2: a big heavy 12-volt hot wire directly from the pickup's battery to the plug, for the trailer battery charger wire, and
3: a 12 volt wire for backup lights if any of your trailers have backup lights. If your truck has backup lights, you can probably tap into the backup light circuit without having to reinvent the wheel. The center wire on the 7-pin connecter at the bumper is almost always used for the backup lights, so if you don't need that 7th wire then simply omit the wire to the center of the plug.
Here is one source for some of this info:
Marksrv.com 7 way wiring diagram page
If your truck has the Ford "trailer towing pkg" that was an expensive option back then, then you probably already have a trailer brake wire under the dash that comes through a relay in a fuse/relay box under the hood. If you don't have the trailer towing pkg, then you probably have some wiring to do. Ford made the trailer tow pkg standard about 2001 model year, but before that you were on your own if you didn't order the option.
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