what is the difference between 83-85 4-cylinder nighthawks?

[Gene]

Besides displacement?

I know there is a:
650
700
750

[mollusc]

Headlights.  Body panels.  Instruments.  Frames.  Internal engine components.
In the US, there was no 750 in 1984-86.

[ExTex]

in 1983, there was also a 550, which was almost a cruiser style. 
And the motor was a sized down 650 engine.

the list:
https://en.wikipedia.org/wiki/Honda_Nighthawk


the 550:  https://www.cyclechaos.com/wiki/Honda_CB550SC

[Larry Fine]

Look for the DOHC version of the engine. Six gears, shaft drive.