Congratulations! My son was 4 years and 4 months when his brother was born. Some of the things we did to help get ready were:
He took a "Super Sibling" class at the hospital and got to tour the hospital and learn about newborns. He even saw what newborns looked like in the hospital nursery.
I took him to an OB appointment to see an ultrasound and hear the baby's heartbeat.
When the baby was born, he got a day off from preschool for the baby's "birth-day". (Baby was actually born at 9:50pm but he got the following day off of school). Son #1 was staying with my parents and he helped my mom make cupcakes for the birthday party at the hospital and he made "goody bags" for the birthday party.
I bought him a "big brother" t-shirt that I wrapped and took to the hospital ahead of time - we told him it was a gift for him from the baby. When he came to meet his new baby brother (we didn't know what we were having), he got to hold him.
We had moved Big Brother to a big boy room for his 4th Birthday in April - and baby didn't come until August. We didn't tell him about baby until June.
I have continued to have Big Brother help me - he planned baby's baptismal party, he planned baby's first birthday party including picking out the "Pooh" theme. The more involved I keep him, the better he is as a big brother. #1 and #2 adore one another and are in love with one another. #1 "reads" books and is teaching #2 songs and games.
I would suggest that you let your 4 year old pick out all of the new stuff for the baby now - but have it blend in. Have her help you arrange the furniture and ask her how she wants it to be...as long as it is reasonable, do it her way. See if you can do matching bedding - or maybe just matching comforter / bedskirts. Try to not "inconvenience" her more than needed - maybe baby can nap in your room so that 4 year old can play in her room when baby is napping.
Good luck! C.