The new standard for UltraHD Blu-ray has been announced, but not completed. That is expected by the end of the 3rd quarter, 2015. http://www.myce.com/news/4k-blu-ray-format-confirmed-72606/
The new 4k discs and players will not arrive until Christmas season of 2015. So, you have a year before shopping for them. 4k TV’s are already out of course.
Regular 1080p Blu-ray will still be made, and standard definition DVD’s will also be available. Its not like these formats are going to disappear.
Jdobbs has not said anything about support for 4k, in fact, he has announced that he is cutting back on support for BD Rebuilder, and does not expect to do much more with it. He may decide to release a “final” edition of BD Rebuilder soon that has no time limit on it.
If he does continue to work on encoding programs, I suspect he will make a commercial version instead of a free one for 4k and H265.
I’m afraid I haven’t heard of your Pivos media center, so I cannot comment on it.