demo, MySQL - rextester

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#MySQL 5.7.12
#please drop objects you've created at the end of the script 
#or check for their existance before creating
#'\\' is a delimiter

select version() as 'mysql version';
DROP TABLE IF EXISTS yourTable;
CREATE TABLE yourTable (postcode varchar(6), ip varchar(55), datum datetime);
INSERT INTO yourTable (postcode, ip, datum)
VALUES
    ('1651ZX',    '8X.26.1X4.9X',     '2017-03-02 22:18:02'), 
    ('1028HN',    '84.26.104.9X',     '2017-03-02 22:19:32'),  
    ('1871EN',    '84.26.104.9X',     '2017-03-02 22:41:47'),  
    ('1651QQ',    '84.2X.104.9X',     '2017-03-02 22:46:35'),  
    ('1651ZX',    '188.206.X1.10X',   '2017-03-02 23:27:07'),  
    ('1111AB',    '217.110.X01.13X',  '2017-03-03 09:12:19'),  
    ('1871AA',    '188.206.X0.22X',   '2017-03-04 21:48:34'),  
    ('1028HN',    '188.206.X0.22X',   '2017-03-04 23:42:19'),  
    ('1823KA',    '84.105.X01.1X',    '2017-03-09 12:28:06'),  
    ('1871EN',    '84.105.X01.1X',    '2017-03-09 12:29:13'),  
    ('1823JM',    '84.105.X01.1X',    '2017-03-09 12:29:40'),  
    ('1818AJ',    '84.105.201.14',    '2017-03-09 12:30:50'),
    ('1651ZX',    '84.105.201.14',    '2017-03-10 09:17:38');

SELECT t1.postcode, t2.post_count
FROM yourTable t1
INNER JOIN
(
    SELECT SUBSTRING(postcode, 1, 4) AS code,
           COUNT(*) AS post_count
    FROM yourTable
    GROUP BY SUBSTRING(postcode, 1, 4)
) t2
    ON SUBSTRING(t1.postcode, 1, 4) = t2.code


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