here are a few great "Big Brother books"
"I'm a Big Brother" by Joanna Cole
"My New Baby" by Annie Kubler (this is a board book, great for a 2 yr old)
"I'm Going to be a Big Brother" by Brenda Bercun & Sue Gross
there's one more I liked called...
"My New Baby And Me: A First Year Record Book For Big Brothers And Big Sisters", by Metropolitan Museum of Art and Marie Madel Franc-Nohain
As far as your 4.5 yr old, theres a book that might help called...
"Making Me: The Pregnancy Activity Book for My Big Brother or Sister" by Julie B. Carr and Josh Cohen
"Baby on the Way" by Marth Sears & William Sears & Christie Watts Kelly...William Sears is a pediatrician, Martha Sears is a registered nurse, certified childbirth educator, breastfeeding consultant ect..., and Christie Watts Kelly has a degree in elementary education, also a prenatal yoga instructor, cofounder of Attachment Parenting International in Memphis. Just trying to give you a little background on this last book's authors so you can choose books with an educated decision.
I have four young children, and each time we would find out we were having a new one, I would break out the books, but I also bought them a brand new baby doll all their own! Yes even my boys! I was sure my DH would have a huge problem with giving the boys a baby doll, but I found baby dolls wearing blue, and Zach (my 4.5 yr old) and I carefully chose a name for his baby. He said his baby is a boy, but his baby's name is Patty. The important thing is, he gets to change his baby's diapers, feed his baby, put his baby nite nite, and love on his baby when pretending Patty's crying. It's amazing what that did to help prepare him for his baby brother. No it didn't "punk him out" like my DH said it would, tee hee.
I hope some of the resources help your little guys prepare for bb # 3, and consider getting both of them a baby doll of their own, you'll be suprised how much that helps.
Good luck, and healthy baby prayers to you,