Hmm...Do you have it set to automatically update your apps or do you manually update them? If the latter, you can possibly figure out what app was last updated before you started to experience lagging issues. Might need to do some investigative work.
Are all your apps from a trusted source? Did you install any apps during the time it started to lag? How much internal memory do you have left? If little, delete some apps to clear up some space. Have you tried CCleaner app (or similar optimization app) to optimize your phone? Have you run a virus scan?
The OS update I just got last week for my Galaxy s7e phone optimized my phone. Did you get an update yet or is s4 not supported by Samsung any more?
Good luck!