Half a crown was commonly called half a dollar, and five shillings a dollar. I've even heard it in rhyming slang called an 'oxford' (scholar).
It's a long time since you got $4.00 to the pound. Today it is $1.25 or so.
In 1940, the British government devalued the Pound to around $4 for every £1. Two further devaluations occurred in the 1960s before the Pound became a freely floating currency in 1971.