Does the Virgin Media Hub 4 support multi-gigabit ethernet to allow you to get full speeds while in modem mode?
losing close to 200Mbps from the advertised speeds if this isn't the case.
Yes you technically can, but it's a bit of a faff. Basically you need a switch with two or more gigabit ethernet ports and that supports round-robin link aggregation. The switch will also need a 10Gbit (or I suppose a 2.5 will do) port which connects to the WAN port on your own router, which naturally would need a 10 Gbit interface.
If you hunt around, there is a post somewhere with a link to an article about setting all this up and the equipment required, off the top of my head, I think it was a MikroTek switch and router which was used.
Yes, despite what is often quoted about 'in modem mode only one port works', that's not strictly speaking true - it's just not that easy to make use of more than one of them, and other than the case of the Gig 1 offering on the hub 4, there really is no point in trying to do so.