As the owner of a manual transmission Sonata I can't say what's normal for an automatic Sonata, but I may still be able to help with your question. My minivan will do the same thing. First the torque converter unlocks, which is good for a 300 rpm increase. If that's not enough, it will downshift from fifth to fourth gear (about 3,100 rpm at 70mph). And if that isn't enough, it will downshift to third (about 4,200 rpm at 70 mph). When fully loaded and travelling through mountains, the cruise control will use third regularly. (It is sometimes annoying.) If drive without cruise but maintain exactly 70 mph up hills, it will still downshift to third. However if I drive without cruise and let my speed slip just a few mph, like from 70 mph to 68 mph, it will not downshift. Like the Sonata, my minivan engine gets its peak torque at 4,000 rpm. If the engine is asked to work hard, that's where it wants to be. (It certainly won't hurt the engine.) Based on my car, I'd say it is normal. But if no one confirms what is normal for an I4 Sonata, you can always check with your dealership.