how do i delete photos from my iphone 4s

For photos that were taken on the phone you select them and then tap the trash can icon.

If you have photos that were added to the phone by using iTunes to sync them from a computer, you need to sync again using the instructions below from Sync photos from your computer to your iOS device using iTunes - Apple Support

1. In iTunes, locate and click on your device.
2. Click Photos.
3. Choose "Selected albums" and deselect the albums or collections you want to delete.
   To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
4. Click Apply.

Learn how to delete photos and video synced from iTunes when you can't access the originals.