Aaah 4 yo are lovely at Christmas, they are old enough to do the Christmassy things but young enough to get completely caught up in it.
(Casts a judgy look at cynical 9 yo DD and 11.6 yo DS)
You need a Christmas Eve hamper.There's loads of ideas on here but in esscence:
new PJs pref festive. Primark are good and about £4
new slippers
a Christmas film- i vote Polar Express!
hot chocolate - I buy the Cadburys instant
marshmallows
new toothbrush
an annual
tube of smarties or fruit gums
something nice for the Christmas Eve bath to wind down
Father Christmas's Tray. My DD has an Ikea teaset.And FC has to leave a 'footprint'
Reindeer food -oats and glitter
When my DS was 3 I made an elf (soft toy with a flannel over its head, face drawn in make-up. I made a little hat with a bell.Elf in a cardboard box bed with blanket ( flannel). DS and I went to his playshed after dark with a candle in a jar to see the elf. Next morning he was gone( Before any awkwards questions were asked!).
Your DD could choose a special decoration for the year the new baby came.
One year we put a little teddy bear on every step.Have to be tiny though so you don't trip.And DD kept moving them to the top of the stairs.
We have a fibre-optic tree in the hall, put on a timer. When the tree lights up, it means he's been so DCs can go downstairs.
I give mine stockings in their room now for when they wake.(usually 6.30 am)
If there's any Santa's Grottos ,maybe at a Garden Centre? Better than hanging about shopping centres with an excitable 4 yo and a bladder that feel the size of a thimble !
On the year my DS was born, my DH and I were knackered with a newborn who didn't sleep.I cannot remember the meal.We didn't even open the presents until Boxing Day 
Good luck with DD and DC2!