MySQL 5.7

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CREATE TABLE myTable
    (`col1` varchar(15))
;
    
INSERT INTO myTable
    (`col1`)
VALUES
    ('1-6542835'),
    ('2-46786283x'),
    ('8-478934293'),
    ('12-573789'),
    ('16-5747385z')
;

Records: 5  Duplicates: 0  Warnings: 0

SELECT *
FROM myTable

col1

1-6542835

2-46786283x

8-478934293

12-573789

16-5747385z

UPDATE myTable
SET col1 = CONCAT('4-', SUBSTRING_INDEX(col1, '-', -1))

Rows matched: 5  Changed: 5  Warnings: 0

SELECT *
FROM myTable

col1

4-6542835

4-46786283x

4-478934293

4-573789

4-5747385z