Another valid answer:

    1231
    131221
    132231
    232221

>! Using similar rules as the accept answer.  The first number specified the quantity of digits of the entire set above it and the second number specifies what the digit is.  The accepted answer is a reading of the prior sequence, this answer is a summary of the prior sequence.

Fun fact, coming back to this.  This sequence will hit a point where it will output the same number forever.

    1
    11
    21
    1211
    1231
    131221
    132231
    232221
    134211
    14131231
    14231241
    24132231
    14233221
    14233221