I d/l the trial version of RM4 and when I try to import a GEDCOM from RM3 or try to create a new database, I get an error message:
RootsMagic cannot create file
C:\Program Files\RootsMagic 4\mast.rmgc
Please make sure you entered a legal file name
Just open the Trial and click File Import and let the program find your RM3 file, which it will convert in its entirity and you will have everything you had in RM3 now in the RM4 format. It will not harm the RM3 file. I wouldn't use the GEDCOM approach you will loose some of your photo and document information etc.