Only 4 email messages open, or only 4 photo attachments open out of how many total?
The iPad's Mail application displays all photo attachments inline or viewed in place within the body of the message, which should be automatic when connected to the internet via an available wi-fi network. If you have an iPad 3G, depending on the size of a photo attachment(s) the attachment(s) will be displayed inline or viewed in place within the body of the message, or as a separate attachment or attachments that must be selected for the attachment to be downloaded and opened within the body of the message.
What is available in the body of the message where the photos should be available?