Hi @pidgin
If you check this link and scroll down to No 6 you will see that calls to the 101 police number are free on all iD Mobile plans:-
https://www.idmobile.co.uk/help-and-advice/call-charges
When is your next bill due? Can you wait to download/view it to see what the £4 extra charge was for as these will be itemised?
An AI internet search answer also states:-
To check current iD Mobile charges before the billing date, log in to the
iD Mobile app or My Account online, go to the 'My Billing' page, and view the dashboard for a live update of charges. You can also see "out-of-plan" charges or usage in real-time within the app to estimate your final bill before it is generated.
Your other option is to contact the Live Chat:-
https://www.idmobile.co.uk/live-chat
When you contact the Live Chat :-
Type ‘talk to a person’ in the dialogue box and this will bypass the (un)helpful chatbot.
Please be patient as the agents handle a few conversations at any one time so there will be a delay in connecting with an available agent and also any replies to your query.
Good Luck x