What kind of Font? T1, DFONT, TTF, OTF?
Does the font have the permission to be embedded?
What program is used to view the PDF from the client?
It is Fira Sans, OTF.
Yes--their license allows embedding in a pdf or epub
Client was looking at the pdf in ibook.
The font displays correctly on iphone 6 and 6+, but not on an iphone 4.
Only Acrobat (Reader/Standard/Pro) guaranties accurate display of PDFs. All other applications, like Apple Preview or iBooks (specially on older devices or systems) support only a subset of the PDF standard. Presenting your data wrong with iBook on iPhone 4 is normal and is expected as iBook is NOT a reliable PDF viewer, but it has been improved with newer versions as was done also with Apple Preview.
But PDFs on mobile devices (Android, iPad, iPhone, SmartPhone) are limited and will never allow the same extend of function as PDFs do on computers.