Personally, I think the answer is nothing. I might drape some festive wrap over one or both of my breasts. Which should anger him due to the lack of direct access.
I made the mistake of informing 4yo dd that we won't be buying her baby brother a present this year as he's too young to appreciate the gesture. She, unexpectedly, burst into tears. She offered me her piggy bank and said she would pay for it and this morning I found her wrapping up her building blocks (in toilet roll) as makeshift presents. She is absolutely beside herself at the idea of us neglecting her brother.
So, I have agreed that we will get something for him from her and wrap it up nearer the time. What do 4 months olds like in the way of toys? DD liked my keys and the cat at that age. I've wrapped up the cat before, for kicks. He didn't like it much.
Those terrifying gloworm things that emit hypnotising lullabies and glow like demons. Any good?