The TDA 2030A is pretty much a bulletproof amplifier, and is capable of delivering much, much higher quality sound than a pinball machine needs.
Although specified for 4 ohms, it will run happily - and more efficiently - with higher impedance speakers. I'm sure there's a minimal - and undetectable in a pinball machine - tradeoff in quality. I prefer running the speakers at 8 ohms if I can because reliability goes up. In WPC machines, the audio electrolytics are usually pretty much destroyed. This destruction happens more slowly at 8 ohms.
The bottom line is:
- You can use 4 ohm or 8 ohm speakers in the cabinet
- You can use 4 ohms or 8 ohms in the head. Wiring two car speakers in series is a great solution
For the cab, pick your speaker on efficiency - 93dB or more - and a frequency response rated from 40 -50Hz. Some efficient speakers start at 90Hz, and that's no good.
For the head, you might need to add a series resistor of anything from 8 - 20 ohms to balance with the cab.