Hi Jane

i am also stage 4. I had breast cancer about 6 years ago but last December I was told it had come back and in March this year I was told it had mastizied to my skin and bones. I have tried not to put a number of how long I have left as whilst there might be an average it's not an exact science. 
Unless you've heard those words, stage 4, incurable, it's hard to imagine how you might feel. I have decided to take the view that I am living with cancer rather than dying of it. Not because I'm amazingly brave or always optimistic (if you read my hello post you will see I'm as fragile as the rest of us) but because I want to enjoy whatever time remains. Who knows how this disease will progress, what I will or won't be able to do in a month, 6 months or a year. I make plans but with the understanding that I may not be able to go. 
You will have days of despair but you will also have days of fun and laughter and making lovely memories for your son. 
Hope this helps but just wanted to let you know you are not alone.