4 hours a day seems like a small amount of time, does that turn your water over completely in that time? As for SWG, it depends on things like CYA level, air temps and how much sun the pool gets. I have a solar cover on my pool unless someone is swimming and only get 6 hours or so of sun so I run my SWG 4 hours which works out perfectly. You may need to run more or less. I really like having the SWG unit though.
pool-school/pump_run_time
edit: Dang...you all are quick.