How to organize Qt DLLs with Qt-based dependency

I have a Qt application using Qt 4.8.5. This application is dependent on a DLL that was built using Qt 4.6.0. Let's call it "MyDLL.dll".

I cannot rebuild MyDLL.dll to update it to a more current version of Qt. Since both my application and MyDLL require the QtCore and other DLLs, and the versions are different, how do I organize my files such that they don't conflict?

MyDLL is required at startup, so I can't use any delay load methodologies.

edit: To be clear, this question came up because the MyDll.dll was built using a custom Qt 4.6.0 from modified source, and my application is using a custom build of 4.8.5. The modifications weren't necessarily the same in both versions, so I didn't want to assume that I could still use the 4.8.5 DLLs. Turned out that it worked out in this case, but the question stands.

Solution

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!