To put it really simple at least for my trains (The R38, R33-ML, and soon the R33-WF [in cooperation with Mr. Railfan] ):
There will be BVE4 versions of my trains once somebody converts them to run at their fullest potential in BVE4. I can not develop stuff for BVE4 because none of my computers support it (I tried running BVE4 by dual booting windows on my mac but it wouldn't talk with my graphics card right
) .
In the near future it is quite likely that there will be a rise in the amount of OpenBVE-only trains for a variety of reasons, most of them due to the obsolescence of BVE2 and BVE4 compatible hardware.