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