Apart from Hagi, I haven't seen much in Yamaguchi that would justify a big detour. Take the bridge at Iwakuni for example: I saw it on my way down the coast and felt that it was a nice little diversion on a long road trip, but if I'd made a special journey to see it, I'd feel let down.

I think Hagi is a lovely little town with old samurai houses and it gets fewer tourists than it would if it were more accessible. But I'm not sure if it's the best investment of your time if you're based in Hiroshima. (It's about 3.5 hours from Hiroshima). If you decided to visit Hagi, you could also stop in Shimonoseki for some Fugu - Feb 9 is Fugu day, which is a big deal there.

If you were interested in Hagi, I would say that it would be better to do it as part of a trip along the San-in coast (visiting Izumo, Iwami, Matsue and so on) rather than trying to fit it in with a trip to Hiroshima. It's worth noting that the Sanin coast has more rain and harsh weather than the Sanyo coast.

A lot of Yamaguchi is a forgotten rust-belt region. If you're interested to see a post-industrial ghost town, I think Ube is a good example. But most tourists aren't very interested in abandonned industrial plant and derelict entertainment centers.

Bottom line - For a couple of tourists based in Hiroshima for four days, I think Yamaguchi offers less value than the places that I mentioned in my previous post.