Moving internally - how to bring this up with manager?

Let me start by saying that I am mostly happy in my role, the people I work with are nice, it is flexible and I work from home half the time. I have always been positive in every 1:1 and never ever mentioned any desire to leave. My role however is boring, easy and slow. My boss is the most accommodating person I have ever worked under and it makes for a very pleasant working environment so really I could go on like this forever, but I also don't want to? I want to learn new things and make a change, have some challenge!

Now, I was looking at the vacancy portal for a friend and saw a part time position advertised which looks really good. It is 3 more hours a week ,which actually is ideal, it pays £2.5k extra a year as well. Trying to find good part time jobs at the hours I want is insanely hard and this is the only vacancy I have seen like this in years at my company. I feel I would be very disappointed in myself if I do not apply.

How do I raise this with my boss? The closing is in 4 days, so I do not really have much time to tell her before I apply and defo not able to face to face until 2 days before they interview...