Can i watch Skins（スキンズ） S3 & S4 without watching the first 2 seasons and to actually understand what's happening?

Yes あなた can. There aren't many references to the first two series in the 3 &4 series. あなた will be able to follow what is going on.

There is one thing that is mentioned in an episode in series 4 that happened in series 2 but it explains what it is.