I always love it when some think their answer is the only.
When some think only in 2 dimensions, I look to 3 for another solution. When others think only in 3 dimensions, I look to 4 for another.
You fail in creative thinking because you limit yourself to assumptions that are not stated. You think in one dimension. I chose to view the riddle in two dimensions. Your rows. And my columns.
You may wish to add more rules to the problem but that would be changing rules to give your answer an advantage after others surface.
Here's another hint: You don't have to add to more than 4 to solve the puzzle vertically.
If I asked what is the next number in this sequence: 1,2,3,4. I would expect an answer of 5.
1
2
3
4
?
Answer is still 5