with a 4th gen CPU you'd be looking at the used market for a CPU upgrade if you stuck to the same motherboard, which would mean ddr3 ram would still be required. If you wanted to go for modern replacements you'd be looking at replacing the CPU, motherboard and Memory as a minimum.
But honestly 100% cpu on a 4th gen I5 at idle isn't right, there might be some software installed thats grabbing the CPU usage. I'd suggest this as a start, do a clean boot up of your pc, dont open anything other than task manager (its on the ctrl+alt+delete screen), sort the CPU column for highest usage and watch if for 10 minutes. If its sitting very high check what applications are using it, and if possible post screenshots of it.