I think the answer is simpler than it seems
Victorian copper pennies and Halfpennies from all had serif 4s up to 44, where the serif was only used for half the coins struck, both serif and plain end. After 44 all 4s where plain end, the next example where 4 was the last number was in 54, these have crosslet ends, both Halfpennies and Pennies.
It could be that the 4 being the last number in the date was subjected to more stress and therefore damaged, a more sturdier design was used, the crosslet, and used in subsequent date ending in 4