Qt 4 and Qt 5 are not binary compatible, so no, you can't use the widget directly. Using both Qt 4 and Qt 5 inside the same application sounds like a dangerous idea...
The best bet is of course porting libpoppler to Qt 5 (have you evaluated how complex such a port would be? Probably not too much). The second best would've been using QX11Embed, but those classes are currently missing from Qt 5, awaiting for someone to port them to QPA / XCB.