This is going to be a very individual choice based on what you use your watch for, how well you see it
, what you do with the new added features, if the latest tech is something you value, but most importantly is your individual financial situation.
If having expanded health data history is important to you, if having the bigger better brighter screen is desired (in the case of my aging eyes it is needed), if newer tech/models is something you’re enthusiastic/passionate for, and several hundred dollars of discretionary money is comfortably spent? I’d jump on buying the new S8 at the substantial discount you’re receiving. But that’s me. I’m enthusiastic/passionate about new tech especially Apple tech and my discretionary funds are decent (I budget my discretionary for new Apple tech). If I could buy the S8 at your prices I’d have bought it in a heartbeat.
But if you use your watch for very specific things only such as tracking workouts or movements, message-phone-Home notifications, the S4 screen size and clarity as is is good for you, if you use an app or two and it works well, ultimately what you use it for gets it done well — and especially if discretionary money is tight??? I’d keep the S4.