Sneezing can be from environmental causes, such as dusty feed or dusty coops. It can also be respiratory disease. If sneezing is your only symptom (of a respiratory problem), I'd not get worried yet on that.
However, if you just acquired a new chicken and you don't have him in quarantine and he is the one sneezing, I'd quarantine him immediately.
Now as for the diarrhea in a 4 week old chick with a flock death- I'd be suspecting coccidiosis.
http://www.clemson.edu/public/lph/ahp/disease_links/images/coccidia.pdf
Treatment is Corid or Sulmet. Corid is better as it treats more strains of it. Sulmet is harder on their systems as well.
I give Corid 9.6% liquid (available in a powder with a different dosage also-in the cattle section of the feed store) - 9.5 ml per gallon of water, new solution mixed daily as their drinking water. I always treat all the chickens who have been exposed to the poo of the ones ill.
Don't administer vitamins with Corid, as it is a thiamine blocker.
Coccidiosis is a fast killer of chicks usually under 8 weeks of age and is an emergency.