/\G(?:.{4})*(?=fred(....))/sg

[download]
will work if you're okay with overlapping matches. If not, you could resort to using the slower 
/fred(?(?{ pos % 4 != 0 })(?!))(....)/sg

[download]

In reply to Re: A regex that only matches at offset that are multiples of a given N? by ikegami
in thread A regex that only matches at offset that are multiples of a given N? by BrowserUk

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