I think they should make Bria v5 work. It’s not like they try to sell you v6 now.
“If they can make 5 work with High Sierra, they can make 4 work with it.” How do you know? Sometimes it’s a lot of work to ensure compatibility. And sometimes it’s too much when ther’s already a newer version out. The effort should go into making v5 compatible.
According to CNET Bria v4 was released on May 10, 2014. That’s over 3 years if you bought it when it was released. If you bought it later, then you have surely taken into consideration that there was going to be an upgrade at some point and you did purchase v4 anyway. You could have waited also for v5. It was your choice.
I think what you demand, keeping legacy software versions up to date, leads straight to software subscrptions which would be even worse. You should consider v5 as compatibility upgrade for High Sierra and be fine with it. Or go look for another solution. You have both those choices.