Auto Correcting and Auto Filling Cells

Hi; if users press the number 4 and then Tab, Enter, or Arrow, how can
Excel auto-fill the cell as "444." However, if users manually key the
complete 3-digit combination 444, Excel should retain "444."
Similarly, if users accidentally type four or more 4s (4444), how can
Excel correct this and change the cell to "444."

Currently, cells in this range are validated through a v-ref list--can
accept 0, 111, 222, 333, 444, and all possible 3-digit combinations of
the numbers 1, 2, 3, and 4.

THANKS!