Why not set up a table of values for this with two fields, one for the short value and one for the long?
The field with the short value would store "4MO" and the field with the long value would store either
"4MO-Hwy4 and Morello"
"Hwy4 and Morello"
A 2 column value list can then list values from this table with the short value as "field 1" and the long as "field 2".
A script could be set up to loop through a set of newly imported records to find any instances where a new short value was used. The script could then create a new record in your table of values and enter the short value, but that would still leave the need for setting up the second field with the long value and I'm not sure if that data is also being imported or not here.