You need to store the return interp value in a variable; otherwise, it will process the first value from 0 to target and next frame it’s going to do the exact same thing, starting from 0 again.
Or:
If I understood you right, you want to increase the value every 4s but keep the interp as well in order to reach the newly set target quickly once the desired angle changes? You want to interpolate because you need a smooth animation, right? Could you confirm?