Cipher recommendation for known-plaintext attack I need to encrypt a 4-digit number. The cipher-text can be up to 11 digits long and there are no limits for the key size. 

Considering that the attacker has access to both the cipher-text and the plain-text but I want the key to remain secret, what algorithm would you recommend?
AES-128 in CTR mode would be more than sufficient. There is no efficient known-plaintext attack publicly known for it. Many others would also do the job.

However, the secrecy of the key depends primarily on the key itself. It should be sufficiently large and sufficiently random, so that it can't be brute-forced.