Simplify the following text for a Grade 13 native English-speaking person.
Original: A Democratic redistricting plan after the 1980 Census carved up the 4th District , with only about 15 % of its territory being retained and added to various territory from other districts ; Derwinski and fellow Republican congressman George M. O'Brien were placed in the same district , and O'Brien won the 1982 primary on the strength of having more of his previous district included in the new configuration .
Simplified: A Democratic redistricting plan after the 1980 Census split up the 4th District , with only about 15 % of its area remaining and added to various areas from other districts ; Derwinski and Republican congressman George M. O'Brien were placed in the same district , and O'Brien won the 1982 primary on the strength of having more of his previous district included in the new change .