The NSA keeps encrypted data (e.g. HTTPS traffic) for five years in the event that they might be able to decrypt it later if they need to. This is extremely likely with respect to HTTPS because the keys are owned by the ISPs who can be directed to hand them over, if not today, then at least when their Certs expire and new keys are generated. So HTTPS traffic is effectively as open as plaintext.

Your 4096-bit-encrypted email would probably not be crackable. However, the NSA would know from the metadata that your mails were "of interest" (because they know your correspondent was someone on whom they are trying to build a case) and they can come to you and demand the key, and impose secrecy on you making it a felony if you tell anyone you had surrendered your key.