Hi Michelle,
Some days I think the dice gets stacked against us, often to the point that we ether have to give up or start looking at the alternatives.
Looking at this image, the mere fact that you were trying to make the image at 4000 ISO at 1/25th of a second at f/5.6 suggests you were on the edge of what might be possible in low light.
Add to this the fact that the sharpening has accentuated the noise in the image and I wonder if there is another way you could have made this image.
There were a couple of ways you could have resolved the low light problems.
First, a faster lens like a 50mm f/1.8 lens would have helped you get more light into the image. With a faster lens you could have been making the same image at 1600 ISO at 1/60s and f/2. Yes, the depth-of-field would be shallower but I would use the opportunity to come back a little and include some of the environment.
The second option would be to ask the subject to meet you the following day at a time and place where you would have more light to work with. This same image could easily have been made in some shade and chances are you would have been shooting at settings closer to 1/250s @ f/8 and 400 ISO. At these settings you would have had less noise and a sharper shot.
Hope this is a help!
Cheers, Anthony