Hygrozime 4 weeks out of date

Yes it is fine. As long as it has been closed and stored well it will still be okay. Sell by dates mainly help keep stock fresh in stores. The product is generally still good for some time after.

i'd look up the individual molecuels on ingredients list... could get good info that way. - enzymes may enature over time or if heat is applied, etc......probably worst-case is it's slightly less effective than before. and debatable how effective it was to start anyway. more likely to be helpful for a soil grow than a soilless/hydro grow.