I honestly don't know if DVRs are very popular in Japan or not. I've tried to find out a few times and they don't
seem to be, but my resources on this aren't the best.
With the MEGAMAX Movie reaching #1 at the box office (IIRC) and the Fourze Driver being in low supply compared to the high demand (Which means the toys are selling well), how can it get the said low ratings?
It is not at all unusual for a Rider's movie to do well while the actual TV show or merchandise line is struggling. This phenomena goes all the way back to Hibiki, where it influenced the infamous staff change-over at the shows 2/3 point.
Hibiki's movie used a separate producer who managed to make the film (and, it seems, it's merchandise) successful on roughly the same level as other Rider movies. Meanwhile, the TV producer was bringing in some of the worst ratings and toy sales the franchise had ever seen (at the time).
Different segments of a Rider franchise serve different audiences. So the people buying belts may not be interested in watching a TV show every week, but may still be interested enough to go on an outing to see Megamax (possibly with their families).
It's worth noting that last year, OOO managed consistent strong box office for movies and record-breaking toy sales, while its TV ratings declined pretty steadily. Fourze's ratings may be affected by the same trend and it may end up performing overall in a similar way.
Does the numbers imply that only a few are watching, or is it not that accurate of a representation of the actual number of people watching the show?
A major caveat to bear in mind when reading Japanese ratings is that most of the numbers we get only represent the Kanto region where Tokyo lies. This region contains roughly one-third of the Japanese population and how a show performs there is usually consistent with its performance in other regions.
Still, it's not too unusual for shows to chart a bit better in the rural regions. The thing to remember there, though, is that all of those regions have drastically smaller populations than Kanto. So if Fourze is pulling a 7% share in one of them, that still probably represents far fewer people overall than a 4.8% in Kanto does.