I want to access an iphone 5s backup, containing photos, on another device. Can I access these photos on my iphone 4, ipad mini, or Windows computer?

If you are referring to an iPhone 5s backup to iCloud, the only thing you can do with the backup is to use it to do a full restore to an iOS device. You cannot view the contents of the backup.

You can view the date, time, and size of the backup from another device using the instructions in the Delete Backups section of the following link (but obviously not proceeding as far as the actual delete step): Manage your iCloud storage - Apple Support

It will restore the information described in the following link (note the qualification regarding photos): iCloud: iCloud storage and backup overview

Also note that doing the restore will replace whatever is currently on the iPad.

The following also may be useful: iCloud: Restore or set up your iOS device from iCloud