4 yr old has recent hearing loss (I think!)

[Lorralaffs]

Thought my 4 year old was being oppositional following birth of little sister but now reckon she has trouble hearing following a series of colds...

What's the right course of action to get this checked?

Do I make an appointment with our GP or Health Visitor?

Yes see GP asap. My 4 year old had hearing loss after DS was born and I thought the same thing, when I finally took her to GP he referred her to hospital where the specialist found she actually had a bead wedged in her ear! Whipped it out and she was back to normal. Now she ignores me out of choice rather than deafness!
Hope you get your DD sorted soon, sure GP will refer her for a hearing test.

Try whispering "Do you want some chocolate?"

I have had moments of frustration with my 5yo DD when I thought she was deaf too but found she just has 'selective' hearing!!

Amazingly she always heard perfectly when the c(hocolate) word is mentioned!

That's why I think something's not right.
I did just that with no response! And "shall we go to the pictures?" - which is normally guarenteed a response.

The colds may have caused glue ear which needs check out and getting sorted. Take her to the gp.