I agree with mcgraw.
When DD was 4, I had DS, I would say things to her to try and keep them ok with each other, it worked, they are really close now (DD7DS3)
Every time you get 5 minutes, give the elder child a cuddle, if baby cries, involve elder child (EC) in the whole 'Oh, here we go again, babies crying, spose I better go and sort it out, what d'ay reckon, feeing again or a horrible yucky nappy' (pull faces and stuff)
Then tickle the tummy of EC and give a kiss or soemthing, do not just get up, leave EC and go off to baby.
Make sure you talk lots about 'when you were a baby' to EC, a child of four is old enough to communicate pretty well tbh, so it should be easier than with a toddler.
Try not to change anything with EC's routine, don't start a new out-of-home activity, they'll likely feel pushed out, maybe make bedtime a bitlater so they havesome one-on-one time, and get as much help from them as possible, 'can you get me the nappy', sing to baby for me whilst I have a wee', etc etc
Most of all just remember EC was the only child and now some little screamy boring thing has come and taken his/her parents!!