Yeah, qt5 should be fully compatible with qt4, there is no much changes, btw there should be easy switch to openGL rendering instead X11 ( in qt4 ) but I doubt if there would be any benefits from it, seems both are accelerated by hardware, no idea how under windows.
On other hand I've read that sdl 2.0 is coming
http://www.phoronix.com/scan.php?page=news_item&px=MTE0MDU

With new client written from scratch we have an good opportunity to do things in a new way compared to existing clients. From the talks in several forums, one thing that always comes up, is how freeciv's 2D sprites based graphics turn most users out, only some playing for nostalgic reasons ("freeciv is like games were in my childhood"). So I could consider even 5.0 as minimum Qt requirement once it (Qt5) comes out. Reason being that Qt3D will be part (not just extension) of it http://labs.qt.nokia.com/2012/04/11/qt-3d-and-qt5-qt4-news-and-releases/ Maybe it would be useful for us?

Just a thought, but given that given that 5.0 is planned within next 3 months, even if it turn out to be a disaster (due to excessive qml or otherwise) and that qt client wasn't functional till now, is there a point of targeting anything below 4.8 ?

That placeholder isn't much useful, I'll remove it in next patch, ok ? So maybe no need to increase qt version.
( Meanwhile I fixed map view update - there were some glitches on down of screen and is working now with isometric tiles, + some fix in tax rate dialog and completed first page of city dialog, I'll post it probably tomorrow )

Split out from patch #3367, based on a post by cazfi to freeciv-dev.

The newly fleshed out Qt code uses at least one feature from Qt 4.7, so needs a configure check. The attached patch from cazfi adds one.