There are plenty of 3rd party routers available, starting at about £30, that some would say are better than the smarthub.
So if I went back to the HH4 I would have to get myself a modem.
Can you suggest any that might be worth looking at, should it become necessary?
Why go to the extra hassle of finding space a modem, when its easier just to replace the HH6 with one of the many third party routers that are available?
If you want to try a modem, then you would be best looking for a second hand Openreach one.
If you want to try a basic third party router, then look at item 5 on the Forumhelp website.