Mine are 5 and 4 now (a little more than a year apart) and it is GREAT! But it was a challenge in the beginning. If you can, try to have some help when the new baby comes. Make sure your husband takes some time off from work if he can. Don't be afraid to take people up on offers to help with meals, babysitting, laundry, etc. I declined some of those offers because I thought I needed to be superwoman, but in hindsight, I should have accepted the help.
Having them close has several benefits: the one year old often doesn't know to be jealous, you can reuse your baby stuff which is still current, they will be close enough in age to be interested in many of the same toys/activities at the same time. If you are lucky, they will grow up being close friends.
Some things to consider that really helped us:
Make an effort for each parent to spend individual time with each child. This will be a challenge, because there is a certain aount of chaos that comes with having so many little ones.
Make the baby wait sometimes. If the youngest was crying but was essentially ok, we would sometimes say out loud, "Now, youngest, you need to wait for mommy. It's older child's turn." It helped the older ones to feel that the baby wasn't always getting first dibs on everything. Also, the kids now take turns really well. No one got used to being catered to all the time. They key is to make sure you do this at a time when you know the baby's needs have been taken care of but he/she is still crying for some reason.
I also talk to the kids a lot about how special they are individually and in relationship to each other. "There's no other child exactly like you." and "You are so lucky to have a sister/brother because . . . " "They are so lucky to have you for a sister because . . . " We reinforce that our family is important. We try really hard to keep any jealousy to a minimum, and I think it has worked most of the time.
Also try to squeeze in couple time with your husband. For the first year or so, it is extremely hard to do this because it gets expensive to find babysitters who will watch an infant and a toddler (and in your case an additional preschooler). It can be overwhelming for a teenage girl and intimidating for relatives. But you can make it work with planning. We had good luck if we asked my mom to babysit for a short time while at least one child would be napping.
It is definitely a challenge in the beginning, but remember that the chaotic time is temporary. As the kids grow older it gets a lot easier. And having them close together means that they always have a buddy wherever they go. I would not trade having my kids so close together because it is truly magical watching them help and love one another. We didn't plan it that way, but it turned out to be better than we ever could have imagined.
Best of luck to you!! It's going to be a wild ride, but well worth it!