If your Prophet's voices 1-4 don't seem to be working but voices 5-8 do work, it's probably one of two things: You're plugged into Output B instead of the Main Outs or Poly Chain is ON.
To confirm, check the audio cables plugged into the Prophet. You can also look at the voice LEDs near the volume knob, see if all 8 of them light when you press a key repeatedly (can't be a Unison/Stack/Split program). If all 8 LEDs light, it's likely you've plugged your audio cables into Output B instead of the Main Outs on the Prophet.
Most of the time you'll likely want to use the Main Outs alone. The outputs work like this: if audio cables are only plugged into the Main Outs, all 8 voices go through the Main Outs. Output B divides the voices for separate stereo processing of stack/split programs. If audio cables are plugged into Output B, voices 1-4 continue to go through the Main Outs but voices 5-8 get routed to Output B. So, if you only have one set of audio cables plugged into the Prophet and they happen to be connected to Output B you'll only get voices 5-8.
If not all 8 voice LEDs lit when a key was pressed repeatedly or there was a gap where some notes were seemingly "missed", it's probably because Poly Chain is turned ON in the Global menu. Poly Chain is for connecting another Prophet (or Mopho or Tetra) to your Prophet, and should be turned OFF for normal, stand-alone operation.