I am fortunate enough to have some access to BT Broadband via a nearby hotspot at a Caravan Site. Can I configure a spare HH4 to act as a repeater to boost this Hotspot Signal to make it more accessible?

No. The HH4 will only accept a broadband signal via the xDSL port or the Ethernet port.

A WiFi portable router with a Hotspot mode would be the answer. You will still be limited to the bandwidth of the Hotspot signal at the point of reception on the router.