I haven't done a fresh installation of Qt myself in quite some time, but I think it would probably be simpler to just download and install the full Qt 4.8.x SDK. I think the MinGW version of Qt comes with its own copy of MinGW, so that makes things easier; if you want to use visual studio, you'd probably have to install that yourself first.

(Hmm. I've gotta admit, I'm using an old Qt installation that I've continually updated via the maintenance tool; there are bits and pieces of it that have version numbers that match the 4.8 tools, but lots of 4.7 pieces seem to be hanging around too. I'm fairly sure that the 4.8.x SDK will build against the Maemo toolchain, but I can't swear that it will; if worse comes to worse, they've still got the 4.7 SDK available from an archive...)

But yeah, once everything is in place, you can then use the Qt maintenance tool (in package manager mode) to retrieve the Maemo toolchain.

EDIT: Pichlo beat me to it. 4.8.x should work fine.