I don't blame you for wondering if there is something not quite right about the request - the pdf with all 4 pictures in it should be just fine.
I wonder if the buyer is not familiar with computer usage? We've had a few problems with customers trying to download onto ipads and other apple devices.
If it's the 'apple' problem, we suggest they look at this:
iBooks is a reader for .pdf files that is free from the App Store.
If you install iBooks, whenever you download a .pdf you will be given the option to "Open in...". This saves it in iBooks so you can keep it without having to download it again.
Then, if that doesn't work, have a look at this:
http://www.ipadinsider.com/how-to-open-syn-pdf-files-on-ibooks-for-ipad/(second option looks best)
If it's not an 'apple' problem, it could be something as simple as the buyer not understanding the download process.
I hope you manage to sort things out.
Kind regards, Lyn