The lack of any official reply since 2013 tells me there is no news about any new version. If you look on this website you will see that they advertise the current version 3.5 as fully suporting Windows 10.
So, the current version still works ok. Not sure why you want version 4.0. Do you have problems with 3.5 ?
Version 3.5 still works good, they obviously earn good money with it and they have no reason to release any other versions at this time. That's how I see it. Unusual, yes, but that's obviously how it is. I would personally like to see Radmin Viewer client for iOS, but I guess they won't bother creating that.