I don't think History Part 4 is going to happen. They pretty much buried the original history concept by the end of the SBIT tour, and I don't think there will ever be a DVD/Blu Ray release again. Producing something for their YouTube channel doesn't make financial sense either.
I would have loved it had they gone through with the plan as announced/indicated in late 2004. But they wasted a lot of time with extending SBIT and ME, not to mention that I think splitting the Powerslave-SSOASS era into two cycles was never the original plan. My guess is the plan was originally to have one double DVD for LAD and ME, but they either realised they had too much material for such a set, or that they could sell both separately for more than a 90s DVD would ever make. Anyway, it seems that by the time ME was over the DVD ship had sailed and a 90's DVD was abandoned.
To be fair, both LAD and ME are great if just taken to be DVD releases of the original videos. But in the context of the "History" series they were a pretty big let-down, ME more than LAD.