you really don't want to use 4.8, if you want to develop for harmattan, you will always use 4.7.4, and if you want to experiment (not really develop for Harmattan) then you want to test qt 5.0 and you can find instructions for that in this board.

And if you really want to use 4.8 then just take some current ubuntu 4.8 packages, dpkg-buildpackage -rfakeroot with some alternative prefix path, and then incept those packages to N9. (And even then you most likely lose touch events, unless you merge same patches that 4.7.4 have).