How do I delete over 4000 emails in my all mail folder on Iphone6

If they are in All Mail they are also in each email account's inbox if you have more than one. So open the inbox that is responsible. The next step depends on the type of account:

POP3: Delete the account, reboot the phone, and add the account back.

IMAP, Google, Hotmail, MS Exchange, Yahoo: Log in to the account from your computer or using the web interface for the account, delete the emails there. When you next open email on the phone, they will be gone.

The easiest and least time consuming way would be to log in through safari and utilize the mark all and then delete all function or to do it from a desktop. If that is not possible you will need to tap edit and mark each email and then delete that way. -AJ

Thanks for this. However, it's pretty pathetic that Apple can't do any better than that!

agree, when i saw my mailbox showing 1xxxx unread message, i just want to delete all by one click. can apple review this? my mail is downloading by pop3 and i need to keep those email on server so not workable for me to delete the email account and reset it again.

Agree, I have 7000 emails. It will take me forever to delete these. If I delete the entire inbox and re-install it, the account just re-downloads all my emails when I re-install. It takes up so much space. There must be a solution. I use my phone to check emails when on the run. I have them stored on my mac. No need on the phone.

There used to be a workaround to this wity iOS10, but since the latest update, even that's put the kibosh on deleting emails. There's not even a "Mark All" option anymore.

Does Apple even look at these forums?

No, just volunteers