I grew up never involved in organized sports or activities until High School. We played outside and organized our own games with very little parental involvement.
Now that I am grown I see the good that comes from letting kids learn new skills and find their passion/hobby. We let our kids each pick one or two activities(a sport and a music)...but at about age 8. They don't always choose 2. Yet, during the summer when our time opens up more than we participate in day camps and other activities and limit the organized sports teams. Our kids are 13,10,6.
I don't like hustling and bustling around from activity to activity because it puts a strain on family time,dinner time,down time and homework time. I find everyone is feeling pressured and stressed on the weeks when all the activities collide and then there are other demands on top of them. Everyone feels rushed and tempers flair because outside forces are dictating when and where we need to be.
We just try to take inventory often on how the kids are doing, how peaceful our home feels and then make decisions to first and foremost protect the peacefulness in our home.
When I had a newborn then we definitely let up on our outside commitments...and my kids tended to enjoy that time most. Meant for more cuddling and reading, playing outside with friends and endless hours of Lego&Bionicle building. And baby was able to sleep in own bed and nurse when needed.
I wouldn't worry about resentment of the new baby. Being in a family means give and take. It would be a good lesson for them to learn that sometimes we have to do what is best for someone else.
I think if you approach the subject with them that it is for a short time and that mommy and the baby need to make certain changes for a while then they will be more accepting. Make sure that when baby is napping you use that time to spend some bonding time with the older kids. That time might not be spent on the soccer field with a team and coach...but in your backyard kicking the ball with them and talking about their day,joys,fears,friends etc.
Have a special treat together when baby is napping..make it a special time...and they will feel like it is a special time. A load of laundry can be going while you are playing with the kids. Dinner quickly whipped together with the kids in the kitchen and then send them out to play in the yard while you put your feet up and take a breather and cheer from the patio.
Congrats on baby #4 coming!!