It's not too concerning; most of the electronics from the time can't emit voltages above 4V despite being powered by 5V anyway.
It is a little bit lower than the HCU04 should be capable of emitting when powered at 5V - the datasheet says under load you should see 0.2V-0.5V below the voltage supply, not a whole volt, but ... 4V should still work correctly.
edit: that said, make sure things change in response to pressing the button.
10+11 are the audio amplifier, so the middling voltages and unchanging output is expected (unless sound is playing). The other 5 inverters are used digitally.The second weird thing is the fact that im getting no voltage what so ever on inverter 5 (pins 10/11) is that normal or is something broken there?