Hello Gwynne, I'm Dr. Deb.
I recently came online and see that your question about this 4-week old pup hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
It's certainly possible that this little fellow is a slow developer but I'd be concerned about him if he has such balance issues at 4 weeks of age. Most pups at this age are like his siblings: able to walk and move without falling over. They are starting to bark, wag their tails and maneuver their bodies quite well.
To answer your specific questions:
1. Since your goal is agility, at this point, I'd consider one of the other pups at this stage in time but allow the possibility that he may be fine.
2. Should he "catch up" and appear normal within the next few days or a week, then I wouldn't consider this an indication that he'll have issues later in life. I wouldn't consider this to be a risk factor for future issues, in other words.
But, if this is a birth defect of some sort, it should be extremely obvious at 8 weeks of age, if not sooner. As attached as you may already be to this little guy, I'd prepare myself that he may not be the pup for you, unfortunately.
I hope this helps but, again, I'm sorry for the delayed response. Deb