You shouldn't be having any issues. Qt maintains binary compatibility over minor and patch releases (see http://qt-project.org/wiki/Qt-Version-Compatibility) and an application/DLL built with 4.6.x will run fine when bound to 4.8.x runtime.
- Your MyDLL.dll built with 4.6.0 will run with 4.8.5 runtime DLLs.
- Your application built with 4.8.5 will run with 4.8.5 runtime DLLs.
- Your application will run fine with MyDLL.dll as long as you are using the same interface you've always been using.
If you are seeing issues it is because of something else, and you will need to clarify exactly what problem you are having.
If you are not seeing issues and are just asking preemptively, then 1) just proceed as normal with no special considerations, and 2) you should have tried it first!