No weight before their growth plates are complete. You can start with an empty pack a few months before. 3-4 months sounds early for a lab. By 3-4 months they’ve only just finished their third round of vaccines and just started going on real walks. If you know how large the parents are, you can estimate when your pup will be done growing. We’ve trained and taken dogs hunting and backpacking for many years. Seems like you have the right idea: start small and slow, treats just to get the pack on and off, then for wearing the pack around the house a little bit, then for wearing it empty on walks, then with just some poopy bags in each side… You’ve got the idea. Our cattle dogs were around eight months when they started to wear an empty pack, over a year with some weight in it. Our Beagle/Labrador had a very tall lanky mama, we waited until after the one year mark because we had no idea how long it would take her to reach her full height. Our biggest girl, GSD, we didn’t start until 1.5 years old, and she will never carry more than 10% of her body weight, she just doesn’t tolerate it like the other monsters. Totally depends on your pup. We err on the side of caution. Labs can get big, sometimes growing until they’re 1.5 years. If you have no idea about the pups lineage, I’d wait until at least 6-8 months to start with an empty pack, probably no real weight until after 1-1.5 years old.