Hi T.,
My daughter was 4 when my son was born too. She was sure she was getting a sister, said the baby told her she was a girl! When he came out a boy, I was nervous about how she'd take it, but when she met him the next morning, she came into our house calling, "Where's my brother? I want to see him right now!"
There were never any issues of jealousy or rivalry. I was amazed at how smooth the transition was. I don't think kids believe that you don't love them anymore, that's not what adding another baby to the family is about.
I wasn't big on the "bring me a diaper, bring me a wipe" helper thing. She just enjoyed being with him. She liked for him to sit in the bouncy seat in her room and watch her play. If he cried and she talked to him, he stopped.
I think some things that helped her have such a smooth adjustment were the following. First, I kept her routine the same. She still went to camp and daycare where she could play with her friends as usual. People were surprised that I didn't keep her home with me during my couple of months of maternity leave but a tired mommy and needy newborn would not have anything to offer her, she needed the stimulation of other kids. I also made time just for her - since I was EP'ing (no latch/no suck baby), I could leave my husband with the baby for a couple of hours on the weekend so that I could take my daughter on an outing. I'd have MIL come or a babysitter for an hour after the baby ate so that I could have that time to just do something with my daughter - play in the yard, read stories, do a craft or bake something together. I still did the bedtime reading with her, hubby had to deal with the baby if he was awake then. Find alone time for her/with her. Also, since it often seems like the baby comes first and is so needy, if the baby would start to fuss and I was doing something with my daughter, I'd say "I'm sorry, you have to wait a minute. I'm busy with K right now." Of course the newborn had no idea what I was saying, but my daughter got the message loud and clear - right now, at this moment, YOU come first and the baby does not
Good luck!