dont know, but it may have had something to do with holding the coins in a fisted hand before the "clip" kind of like the brass knuckle effect. This is just my guess and i maybe way off base?
A 'fourpenny one' was probably originally a particularly large size of ice-cream...perhaps a basic cone was worth only twopence. It was certainly used in phrases such as 'fourpenny ale' to mean especially strong beer. As a result - my guess is - it took on the meaning of an especially hard blow from a fist.
The fourpenny bit (coin) apparently had gone out of circulation by 1890. There is no record of the phrase 'a fourpenny one', meaning a blow, before the late-1930s. Half a century seems rather a long time for it to have lurked only in speech before someone decided to write it down, though that may be possible. What seems virtually impossible is that the story in H's link above could be the true source, dating back - as it does - to the 1500s!
Just to say that my grandmother used to use this phrase [she was a Londoner] and her pronunciation of 'fourpenny' was 'fawpny' - nowadays I often hear younger [or posher] people pronounce all the syllables in twopence - it's tuppence - or threepenny - it's thrupny!
And I recently heard a young actor say, 'Well I never did' in a play, with the emphasis on 'I' - it should of course be on 'did' but I think they had not understood the meaning of the words [ie it doesn't mean 'well it wasn't me who did that', but more like 'well I never did hear of such a thing before'].