On Netflix you can watch The Seven Deadly Sins and there are also the 4 OVA's. Can I also skip the four OVA if I'm through with the first season now and want to start the second one now? Or are they very important for the story?
Yes, plays between season 1 & 2.
The 4 OVAs »Nanatsu no Taizai: Seisen no Shirushi« aka »The Seven Deadly Sins: Signs of Holy War« are the continuation of season 1. Season 2 is the continuation of the OVAs. So one part of the story.