Yes, vB 4.2.5 is the last version of the 4.x series that will be offered. Supposedly there will be security patches made available if the need arises. I see we are running vB 4.2.3 here. The main advantage to running vB 4.2.5 is for support of PHP 7 (7.0.x and 7.1.x). I noticed a nice little boost in speed when I updated the vB site I admin to that version, and update our PHP to version 7. I did have to do a bit of work to update some of the older products (addons) to rewrite deprecated functions.
Most people I know in the vB community recommend highly against updating a vB 4 site to vB 5. Most move to XenForo (XF) from vB 4 from what I've gathered. XF behaves very nicely on mobile devices too, from what I've heard (I'm a PC only guy). I also like IPS, but XF is more like vB and users seem to have an easier time making that transition.
Over at TheAdminZone (TAZ), vB takes a lot of heat for being out of date, not mobile friendly and for the dismal performance of earlier versions of vB 5, but having worked with vB, XF, and IPS, I still find so many things much easier to do with vB. Sure, a big part of that is familiarity with the platform, but there's been many times when I want to do something, and find I need to write custom code, or purchase an addon, to simply do things vB will do out of the box. Also, vB has a support site that offers thousands of free products, whereas the more modern platforms have support sites with paid addons only. People don't seem interested in helping each other out anymore.
