There is spamgourmet (https://www.spamgourmet.com/index.pl). It doesn't satisfy 4 exactly, but it's pretty close. If you give out an email like 'foo.20.myusername@spamgourmet.com' (there are other domains too), then it'll forward 20 emails to the configured email for your account, but you can always log in and set it to 0 remaining at any point, regardless of how many emails have been received.
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