Hi Loritx.
I am certainly not a turtle expert in any sense, however, I have learned a few things from my neice and my turtles. At 4 inches or so, your turtle is surely pretty young. At that stage, they do prefer a more meaty diet. I would recommend watching where he/she prefers to hang out for sunning, and set a few mealworms (petco, petsmart, etc... they need to be refrigerated) there. Turtles love these things. I used to hand feed mine, but they were getting too close to my fingers, I now use a fork. You can probably even try earth worms from your yard. They do sell pellets for turtles, but they are expensive, and your fish will gobble them up. Water turtles must be in water to swallow, something to do with not having saliva, or something. Even if the meal worms fall into the water and drown, either your fish or the turtle will eat them. After a feeding, my turtles go crazy searching the pond for any that may have dropped to the bottom. Good luck. I hope this helps.
kamala