Good work so far. The 4-pin connector might be under-hood and i think a likely source of the trouble. In the last 45 years somebody may have pulled it apart -- i seem to remember it is the test connector for the ignition

it may also be covered with tape. Are there any bulges in the harness under the hood?

at a guess perhaps somewhere near the firewall or under the black metal box next to the coil

That tach was the hardest thing to search for in the Pacific Northwest. After my car was totalled, I sold it to someone in the Seattle area for $50.
When I pulled it from the junkyard I cut the plug off the wire harness but found out that the plug was just a standard option on the wire harness because car had one.

The guy who owned the car before me had done some weird stuff to the ignition, so I think you're right in that it may be pulled apart right now. I'll check tonight if I see anything under the hood. I didn't see anything when I checked under the black box next to the ignition, but I'll check again to be sure. Currently my dash is all ripped apart but I need the car for tomorrow so if I can't find anything tonight, I need to wait until the weekend. I apologize in advance if I can't give an update for a while.

As far back as I can remember, accessory plugs were wrapped with blue tape to identify them, but I have also have seen yellow tape used, this may help you in locating them.
Also later model vehicles that had electronic ignition used a "tach resistor" in the circut to get it to function. If yours requires one and it's not there the tach won't work no matter what you do. Check a wiring diagram for your year and model to be sure.