If you Google what you want to do there are plenty of guides out there. But first step is to connect directly to the HH4 & change it's IP address to one in the same range as your main router. You haven't said what that is but if it's another BT router it will have a gateway/admin address of 192.168.1.254. You need to change the HH4 to 192.168.1.x where x is any address outside the DHCP range of the main router. I think that's below 64 but check for yourself against what you have. It also needs to be memorable so maybe the other extreme of the IP addressing range, 192.168.1.1, so you can connect to the admin page of each router by using it's unique IP address.
Then turn off DHCP on the HH4 & change the SSID(s) & password(s)to match the main router. This should allow seamless roaming of devices without having to manually switch networks. How well that works in practice depends on the client's ability to release a weak connection in favour of a stronger one.
If you leave all the WiFi channel selection to automatic, they should play nicely with each other. I would prefer to use a WiFi analyser to look at what channels are in use by neighbours & manually select them all. You want different channel numbers for the two hubs, based on what's least congested. Ideally sticking to 1, 6 or 11 for the 2.4GHz band.
The connect the two together using their ethernet ports (not WAN) & all should be good.
This is quickly off the top of my head so as I said, Google & you will find other guides & probably YouTube videos too.
