What is the V4 Mail folder, and can I delete import.mbox

For first half of your question you imported mails into Mac OS Sierra from Mac OSX Yosemite .

An article : Mail for Mac: Import or export mailboxes

So it is reflecting in user library > mail > V2 folder and the old data of mails is there .

And , for the second half of the question , V4 folder has all mail data in its subfolders and alfa numeric folders in Mac OS Sierra .

So , don't delete any of your folder and its subfolders that is in V2 and V4 folder .

The V2 folder is used be Yosemite and shouldn't contain anything used by Mail in Sierra. When you moved to Sierra, everything should have been moved to V4 unless something went wrong.

Before doing anything, make sure you have a current backup.

Quit Mail and move the V2 folder to the trash, but don't delete it. Open Mail and verify everything still works OK and all your messages are there. Once confident everything is OK, you can empty the trash.

For the Imported mailboxes, they shouldn't contain anything used by the main mailboxes. If they show in the sidebar, delete them from there.

V2/3/4 is a folder that the Mail app relies on in order to figure out how many emails you have, in what mailbox, from which account and/or if rules are applied.

Deleting or altering those folders may not harm your mail app at all, if the change that was made was insignificant or on a folder that you don't care about.

Deleting or altering those folders may also be catastrophic for the mail app as it could "forget" what you had and where.

Hi again.

What I forgot to mention (sorry) is that V4 contains everything that all other folders contain.

So what you can do is to remove V2 and others and place them on an external drive while mail app is closed.

Open the app and check your emails.

If nothing missing, issue resolved

It may be that since you did not use Mail in El Capitan, the folders did not migrate seamlessly and maybe some items are duplicated. But it would seem that all your messages would be under the Imported mailboxes in Mail's sidebar. Did any of your messages migrate to On My Mac, or do they exist in the Import Mailboxes?

Assuming that you didn't use Mail in El Capitan and started again in Sierra from Yosemite, try using Mail's Import function. Move the V2 folder to the desktop. Choose the first option and navigate to the V2 folder. All imported mailboxes and messages should appear in Mail under an import mailbox. You can move them to On My Mac from there and delete any duplicates.

Once you have everything where you want and have all your messages, you can delete the V2 folder and any Import Mailbox from Mail's sidebar.

When you imported mails in Mac OS Sierra , they will be shown in bottom of mail app side bar , you can create new mail box and drag and drop them into new mail box ( name the desired mail box ) , and if V4 folder is twice big then V2 folder , same way drag and drop some mails that are not important for you , from it to another created new mail box .

And if you right click on sent mail > click on get info account the mail box size should not exceed 18 .25 GB .

You can distribute the mails in different mail boxes .

No, the V4 folder should not contain everything the other folders contain. Messages should have been migrated from the V2 to the V3 or V4 folders.