I sincerely hope so.I very much believe Yu can still make a great game out of IV. All the foundations have been laid in III, so all the resources spent learning the engine, developing assets, gameplay, etc could now be allocated to developing the story and giving it more depth. All the groundwork has been done, so IV could very well end up exceeding our expectations.
However, I think a lot of people are overestimating how much can be reused. Unless Shenmue 4 is going to be like Majora's Mask to Shenmue 3's OoT, reusing all of the same NPC's and location assets.
They will still otherwise have to create all new animations for the new move set, new NPC's, new cities/locations, record new dialogue, likely new gameplay mechanics and mini - games (though some could be reused) etc. Even if they can reuse some of the assets, and they will, it will still be a lot of work on a tight budget.
The biggest asset they will have going into S4 is experience. I think a lot of the devs were inexperienced with UE4 and a lot of time was lost to the learning curve. Also, general development lessons learned with what worked and didn't will speed up the process for the next one. If they get greenlit early enough they can use the same dev team who will be fresh off S3 and ready to go.
So in short, yes I think they could be in a better place for the next one, but I remain (very) cautiously optimistic. Hopefully though I'm wrong and it turns out to be fantastic.