Remember, to get reliable transmission, you need frequency within 3% of the "target frequency", so with 12 and 4MHz you will be able only to achieve very low speeds reliably. (2400 baud?) Check if that 11MHz is plain 11MHz or maybe 11.059MHz - the 11.059MHz is used especially with standard RS232 speeds in mind (the dividers allow you to reach many default speeds, up to 56200 baud)