@VanGoSunflowers It's exactly that.
I've found that once they begin to get more confident - they've had enough walks that things like ducks, or bins, or cars, don't scare them, and they've made 'friends' with other dogs and people 'pat them' and are nice - that they get a bit too confident. What provided them with lots of stimulation to start with...no longer does.
It's like reading a book for the third or forth time, you still enjoy it...but it's not exciting anymore. Or it's like going from a child on their first day of nursery, needing that reassurance and finding everything overwhelming, to teenagers who think they know everything overnight and want more than they can reasonably handle.
They also get more energy. They develop that much faster - both mentally and physically (see p.17 of the Adolescent for Twatdog at 4.5 months).
By four months, they've lost or are losing all their puppy teeth and gaining their adult teeth. But they often still don't understand just how powerful their jaws are - Goldies are particularly awful for this tbh - and where their baby teeth just make you go 'ouch' their adult teeth can actually cause damage. So although they're not technically biting harder...they are.
I often think that even though they're not capable of going on hugely stimulating walks or doing extensive periods of training it's what they need it at 4-6 months. But at that point their needs outstrip what they are capable of (if that makes sense) and they need so much more than they can really be given (or handle). It's awful.
It's what makes it most challenging, because you have all that energy and intellect building up and no real way to give them an output...and that ends up with one thing: manic, bitey, chaos demons.