4 to 11 is fine. If your plants aren't doing great then maybe make it a bit longer.I turn my on at 8:30 A.M and off at 8:30 P.M.
Too much light is as bad as too little, then you start encouraging algae. You need to find a balance that works in your tank.
Floating plants are fine, but I'd avoid duckweed as it is very difficult to get rid of.