The children were targeted by paedophile Robert Pearson (67) of Bennochy Gardens.
Some of the offences took place in swimming pools in Dundee and Coatbridge as well as on a beach in Elie.
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One of the victims found it difficult to sleep, suffered nightmares and could not speak to anyone about what was happening to her.
However, she wrote a letter to her teacher and the police were informed.
Pearson, appeared in the dock of Dunfermline Sheriff Court and admitted a string of sex offences.
On various occasions between September 11 1984 and September 10 1985, at addresses in Kirkcaldy, Burntisland and elsewhere, he used lewd, indecent and libidinous behaviour towards a girl, then aged 13, made sexual comments to her and touched her body over and under her clothing.
On an occasion between November 1988 and November 1989, he used lewd, indecent and libidinous behaviour towards another girl, then aged 11.
On various occasions between December 1 2010 and August 3 2017, at various locations including Olympia Leisure Pool, Dundee, the Time Capsule swimming centre, Coatbridge and Shell Bay, Elie, he sexually assaulted a third girl. This girl was aged just six when the abuse began.
On various occasions between January 1 2012 and August 13 2017 and the same locations as the previous charge he sexually assaulted a fourth girl, who was aged between 4 and 10 years at the time.
On occasions between January 1 2012 and August 13 2017, he intentionally caused the fourth girl to participate in a sexual activity.
Defence solicitor Elaine Buist said Pearson had no explanation for any of his actions.
She added that he now lived on his own following the end of his marriage as a result of his crimes.
Sheriff Charles MacNair called for reports and refused bail describing the offences as “a horrendous course of conduct”.
Pearson was put on the sex offenders’ register.
There were cheers from the public gallery when Pearson was led away to the cells and a shout of “beast”. He will be sentenced on January 15.