Our 4 week old lab puppy's nose is very dry, is this normal?

It's a myth that a warm dry nose = sick dog.
Sadly, it's a myth that millions of people still believe.
Look at the real signs your dog is giving you.
Is he playful? Eating/drinking regularly without a problem?
I believe dog's noses are normally wet from constantly licking it with their tongue.
Now, a dry nose could also mean high temperature.
but this may be a simple , non-licked nose.
That's my guess at this point.
Since your pup is sooo young, its hard to really tell.
If its crusty you may want to contact a vet, otherwise, give him some water to drink and see how his nose looks (or feels rather... lol) later on
=)