Q: how can i delete pictures from my previous iphone4 backup on icloud

You can't view the contents of a backup or delete only certain parts of a backup. You can however delete the complete backup of a specific device. If you no longer have the iPhone 4 and you have migrated its contents to your new iPhone 6 then deleting the backup may be something you would want to do. If not, another option is to purchase more iCloud storage. The links below have instructions for both of those options:

Manage your iCloud storage - Apple Support

iCloud storage upgrades and downgrades - Apple Support

Also the iCloud backup is just that, a backup. It's not achieve like Dropbox. If you delete photos from your iPhone 4 the next time the phone makes a backup they are deleted from the backup.