Hi,
In addition to Ludek points and you still want to move DB from one to another MM version then when you move it you should do full text search reindex on each switch.

Are you saying that if you define/change the collection in MM4 then MM5 subsequently changes it by itself, I doubt, I suppose that only after editing in MM5 it exhibits the issue with comma versus semicolon ? In that case editing the collection in MM4 is a workaround.

BTW: Although it works with ForceMM4Compatibility ON it is still not recommended to share the same database between MM4 and MM5, namely editing in MM4 causes that new fields in MM5 (e.g. Custom 6 - Custom 10) are not properly indexed for searching etc. At least be sure that you have backup of your DB.

Since MM4 seems to be end-of-life, I am trying to use both MM4 and MM5. I have the option ForceMM4Compatibility=1 set so that I can send the database file back and forth between MM4 and MM5. Unfortunately, there seems to be a problem when certain properties are set on a collection. For example, if I set my Music collection with the following property - file extension = m4a, mp3 (in other words, the file extension can be M4a or mp3), then it won't properly feed back into MM4 - I can see that there are both a comma and semicolon being inserted between M4a and Mp3 when viewing the collection properties in MM4, which may be the cause of the problem. The collection is not viewed properly in MM4 because of this error. Note: this is a simplified example, in my case I was setting several additional file extensions (aac, alac, flac, etc.).