Yes that is quite normal. I'm not sure if that tech tested the batteries under a load but sometimes one battery looks slightly weaker when idling but drops extremely when put under an electrical load. Typically I see Wave batteries last 2-4 years before needing replacement.

I would check the overall battery voltage of all 4 batteries combined idle (should be around 24-26 volts depending on charge) then check when running the hydraulics to deadhead (as high up as possible) and check the voltage while still running the motor. If it drops more than 15% from the idle battery voltage, then one or more of the batteries is dying and needs to be replaced.

I typically recommend replacing all 4 batteries if they are older than 3 years to prevent any further issues, but it is possible just to replace one at a time.