<p>How can I add a plaintext into encrypted ciphertext?</p>

<p>For example, I would like to add number 4 into encrypted(5)</p>

@Bill the lizard:
Well you can, if you are using ECB or any other block mode where the n+1 block doesn't require much knowledge about block n you can chain on an extra block (or even insert a block as in the case of ecb).
However this leaves you open to mitm-attacks and so on...