I think I have figured out what is going on. There was a time during version 3 releases that the documentation said that the custom tags were not saved in the file but rather in the database (I think). However, I just looked at another PC I have that is running v184.108.40.2064 and in the Help it says basically what the new version 4 is saying. It doesn't say that the data is not saved in the file but it doesn't say it isn't either. So I used Notepad++ (neat program, BTW) to look inside one of the files in which I had placed some of my custom tag info and I found that the tag info is there (done while editing in MM v220.127.116.114) but strangely enough the names for the fields are not. For example, my first custom field was (and is) labelled "Billboard Entry" and I put text dates in this field in the form "yyyy/mm/dd". In the song called "The Rain, The Park and Other Things" by the Cowsills in 1967, the following info had been saved into the file in the id3v2 tag area:
So the first custom field has the text "1967/09/30" and the second custom field has the text "2" (Billboard Peak position). However, nowhere in the file is the text "Billboard Entry" or "Billboard Peak" contained. It only showed up as the title for all songs within MediaMonkey. When I put new titles back in for the Custom Fields into MediaMonkey v4 then everything showed up again in the various displays where I am showing the custom fields and also in the tag editor (it seems like MediaMonkey ignores any custom data until the headings are defined within MediaMonkey).
So on one hand I am thrilled that I didn't lose the info I had entered for my songs.
On the other hand, since the titles are not saved with the file, if I get a song from someone else and they have something else defined for these custom fields then it will show up in my listings in the same custom fields although the data have nothing to do with what I defined the custom fields as. I know that within the MP3 id3v2 specs technically the title of the first field is "XXXSongs-DB_Custom1" and not whatever I have given it within MediaMonkey but it wouldn't be too difficult to have twice as many of the custom fields defined with them being in pairs, the first one being the real title of the field and the second one being the text of that field (I know, that's not a bug & this section of the forum is for bugs so I'll drop that line of thinking here).
Having these custom fields is neat because within MediaMonkey I can right click on the various listings and have one or more of my columns show up and then I can sort on that. For example, it is a snap to show all of the Pop music I heard on the radio that came out the spring of my senior year in high school.
So the bottom line is that a) there is no bug related to custom fields that I have found, b) the old custom data had been written to the MP3 file and was still there, c) once defined in any way, data that has put in these special custom fields by MediaMonkey or any other program will show up as whatever has been defined with the new titles and d) I think the documentation could have been a bit more clear about what was/was not saved into the MP3 tags.