I recently upgraded to the iPhone 5s from the iPhone 4s. The photos I had on my photo stream on the 4s are no longer present on my iPhone 5s. Is there any way that I can retrieve them?

Photo stream photos only remain in iCloud for 30 days (even though your last 1000 remain on your device until you delete them). When you enable photo stream on a new device you only receive photos from the last 30 days, if there are any. Older photo stream photos are no longer in iCloud.

If they were in the camera roll of your 4s when you last backed it up, restoring the backup to your 5s should recover them, along with other camera roll photos, app data, settings, apps and other purchased media, etc.

Sound like, for whatever reason, they weren't added to photo stream or they would have streamed to your new phone. If you backed up your 4s while they were still in the camera roll, restoring the backup to your 5s should recover them.

If they were in your photo stream, and are less than 30 days old, they should stream to your new phone. You could always turning photo stream off, then back on; or try using a different wifi network (just to eliminate networking issues as a variable).

Good luck!