The reason M4 Base flagged an error is it's installer is looking for Poser.exe which does not exist with either PoserPro or PoserPro 2010.  For the pro version, PoserPro.exe is used.  My suggestion: create a dummy Poser.exe using notepad.  Open notepad, save the document as Poser.txt.  No need to type anything.  Change the txt extension to exe.  The icon will change to a Poser icon.  Find your PoserPro folder and move the dummy Poser.exe within.  Don't worry about overwriting the original file, it is named differently. 

Now, when you install M4, the DAZ installer will find the dummy Poser.exe and will properly load the content into their rightful places.  There should be an automatic initialisation at the end of installation. 

The expression inj will not physically change anything on it's own.  You did nothing wrong.  It will create additional dials in the model's properties.  If you have a problem conceptializing the set up, let me know and I will screencap what to look for.

BTW, as a matter of proper version terminology, there is no such Poser version as Poser 7 Pro.  That has been a common mistake by many and it is not correct.  The current version names are Poser 7, PoserPro, Poser 8 and PoserPro 2010.  There will be less confusion when you reference to your particular version.  Now, be careful.  There is a Poser ProPack which was released after Poser 4 and bears no association with the current Pro series.