It looks like a normally-open (obviously) circuit across two of those four pins, when the circuit is closed the unit goes into boost mode. In programming you can set the boost overrun time 0hr, 1hr, 2hr, 4hr, 8hr.
So you could wire a simple light switch to this and have overrun on 0hr, meaning the unit will boost when the switch is on and return immediately to trickle mode when the switch is off. Or you could wire a doorbell button to it to activate whatever overrun time you set in programming.
Quite handy to have but I'm not paying £100+ for that. Bit more info here, but they're getting into a more fancy Zigbee set-up
https://community.home-assistant.io/t/volt-free-relay-for-vent-axia-fan-boost/647242
The OP in that thread was able to bodge a connector but mentioned he thought the proper connectors were JST-GH. That can't be right as the pitch (distance between pin centres) on those connectors is 1.25mm and I've measured the Vent-Axia ones at 2mm...
Some more close-ups of the sockets...
I've done a fair bit of Google-Fu this evening to find the connector. The JST-
PH (shown on the right in the below picture) has a 2mm pitch but the connector shape looks quite different and there is no top-latch variant. The connector on the left is the Seeed Grove and looks close enough for me to chance around a fiver for a pack of 5 plugs with 50cm tails. If it's not quite right I'm sure a fresh Stanley blade will make it so.
Link to pack x5 50cm plugs
https://www.ebay.co.uk/itm/132661875585 50cm isn't going to get me from the PIV unit to where I want the switch to be but I'll splice it onto whatever 2 core I have lying around. 0.5mm² flex would be fine.
I've topped my shopping list off with a branded Vent-Axia boost switch for a tenner on Amazon. Like I say, you could use a basic 1 gang/1 way light switch but this one looks the part
https://www.amazon.co.uk/Vent-Axia-455213-Boost-Switch/dp/B01H3IPZQK and will help convince her I've done it "properly", since this will be the only bit on show.
I just need to find out which of the 2 pins of the 4 are used but I will just cross my fingers and bridge them till the PIV kicks into boost. It begs the question; why did Vent-Axia use a 4 pin connector here? Possibly something better was planned, I think a remote programmer would be amazing because climbing into the loft to make changes is a proper pain. I cannot find any hint that such a thing is/will ever be available though and I hate to think what the price would be if they want over £100 for a simple button and lead, lmao.
The more likely reason is they didn't want people accidentally swapping the heater (240v) and boost (volt-free) plugs.
I won't have time to do this for a couple of weeks so I suggest people hold tight on buying any of the parts I listed, just in case they're wrong. Will report back whether success or disaster.
. Thanks for you time guys