Well - it is simple, but not that simple. You had some good advice already. I have a Garmin 76 and a laptop that is running FUGAWI nav software.
I happen to have two cables, one ends in a "Y" with:
a 12 V plug connector on one side of the "Y"
Its the big thing on the right end of the cable in this picture. For now pretend the USB thing is not over on the left side of the cable in the picture.
and a RS232 Serial port connector on the other.
It is the rectangular connector on the left side of this picture.
The last picture is an adapter cable that lets you plug in the RS232 in one end and plug into a USB on your laptop with the other end.
The adapter can also look like this:
OK - that will get the Garmin "plugged int0 the PC"
From there it take a bit of tweaking. As the other post reads - it does not really connect it to a port, but "emulates" a port. I think those were his words. Again - can be done, easy "when you know how"
This is one of the advantages (some will say the only advantage) of keeping a few pet geek teenagers around to help you. With the Garmin and the RS232 and the USB emulator / adapter thing - the order you plug things in and turn them on matters.
OK - Cable #2 that I have is the cable you describe - four wires.
I have that as well and the red and black are wired into my 12 V power and the other two are wired into my little mini IT system. Over near the chart table is a computer wire with a RS232 end on it which, I plug into the USB emulator /adapter ... you know the drill here.
Easy - ya, sort of
Simple, ya sort of
Do you need patience and a bit of geeky help - well, I sure did.
And for me - once I did get it working exactly the way it should (and it will) I took a pencil and wrote on the fiberglass bulkhead right beside where the computer sits
Plug in sequence - ya da ya da
Comm port set up - - ya da ya da
Baud rate set up - ya da ya da
Cause at my age, I have a good memory, but it is short!
I hope this helps a tiny bit