I use a Fallkniven F2. About 4.5" very nice and sharp VG-10 blade, grippy rubber handle, kydex sheath. It's over $100 and has a stiff, fillet shaped blade, but it works good for cleaning trout. It fillets Ok. An excellent knife. If you wanted something less expensive and very functional, I would look at the Buck Silver Creek line of fishing knives or their new Clearwater line. Those appear to have the flexible fillet blade that you're asking about; however, they are all 6" long or longer, and that would not be the optimal length for cleaning trout. I think you'll be hard pressed to find anything under 6" that flexes like a regular fillet knife. That Fallkniven is the closest thing I've found.