4kWh is only enough to run a (barely adequate) 10kW electric shower for 24 minutes. It seems unlikely that this is 'average' useage for 2 if you add in 2 daily showers plus cooking - maybe if you only shower every 3rd day....!
From the numbers you posted, you are using a reasonable chunk at 18:00 when the DHW comes back on - this is making up only the loss on the tank - so if you want to save some of this, get some more insulation on the tank. Even if you only cover it with a old duvet or similar you will find that it will be hot under the Duvet.
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