Why would my 4 cylinder car suddenly dropped from 70 mph to 45 or slower?

It sounds like a transmission issue, or you accidentally dropped the car into a lower gear.

In 5th gear the cars RPM as shown on the tachometer is normally at about 2000 rpm at 60 mph. IF you drop the car into 3rd gear the engine RPM's must be real high (say 4000 rpms) to maintain speed.

If you're still at 2000 to 3000 rpms the car speed will slow to about 45. So the engine is working harder, but you're not going faster. If you're in cruise control the car has to increase the RPM's way up to maintain the cruise control speed setpoint.

Make sure you're in the right gear. Also, some cars have a button on the shift for overdrive. If you hit the button and take it out of overdrive, it effectively changes the transmission to a lower gear.

Check your spark plug wires, make sure they are all pushed on really good. The same thing happened to me and a miata, i was going about 60 and then it dropped fast and i pulled into the gas station and checked it out. my 2nd cylnder spark plug wire came loose. Just take each one and carefully make sure it is lined up and pushed in all the way. if it still does that, the only things i can think of off the top of my head would be

spark plug wires, spark plug its self, distributor cap or rotor. just youtube how to check it, its really easy. Im 19 and i changed mine before. good luck!