Yes. In Photoshop CS4, try stitching again with File > Automate > Photomerge and enable “Blend Images Together.” That option often smooths exposure differences across the panorama.

If you need to fix the existing pano, use a soft/feathered mask over the darker or brighter side and apply local adjustments. Levels can help match brightness, and if that shifts color or saturation, follow up with Hue/Saturation or Lightness adjustments to rebalance the sky. This is usually less noticeable over detailed ground areas than in smooth skies.

For better results at the stitching stage, use panorama software with exposure blending. Hugin was specifically mentioned as doing a good job of finding a correct exposure balance, and dedicated panorama tools may also use EXIF and lens data to correct issues like exposure variation and vignetting automatically.

In short: re-stitch with Photomerge blending first; if needed, correct locally with masks and tonal/color adjustments; and consider Hugin or another exposure-aware panorama stitcher for future panoramas.