You need to ID the tire size (need the determine the tire circumfirence), rear end ratio and the number of teeth on the drive gear in the 4-speed to determine the gear to use.
Driven Gear Formula ... Drive Gear Teeth X Axle ratio X Tire Revs per mile divided by 1000. (from the Mustang's Unlimiting catolog)
One mile = 5280 feet.
 

The four speed requires a much longer cable and a reverese helix than any other Ford transmission.  That is because the engagement on the transmission is on the passenger side (opposite the three speeds and automatics and T5s)  It will be interesting to see how you install the cable from the top first.

It is not that hard on a 69-70 to reach up there and install the cable to the speedo, even with full dash in place.

You can also use a four speed cable from a 65-66 Mustang.  It has the right length and upper threaded connection you need.

One thing I have learned the hard way about the new reproduction speedometer cables, like Scott Drake...  The outer vinyl coating is so thin and fragile you really need to be careful about knicking it on some sharp edge, or burn it with getting too close to the exhaust.  If you nick it, you will soon have a puddle of tranny grease or fluid right under the lowest hanging part of the cable.  Somehow the spinning cable transfers the fluid up higher than one would think.  I have a nick in the new one on my fastback.  To fix it, I took a 4" long piece of 3/8" rubber fuel line and cut a slit down one side to open it up where it would fit over the cable.  Clean everything squeaky clean, fill the hose with ultra black rtv and slip the hose over the hole in the cable so the closed side of the hose covers the leak.  Then apply a couple of carb hose clamps to hold it in place.  I have done a few cables like that and they never re-leaked.  And once the leak goes away, I quit worrying about what it looks like under there.