OSRM have realesed new version, v5.
And I can't find anywhere, what's new in it? I found new documentation of HTTP API, https://github.com/Project-OSRM/osrm-backend/blob/master/docs/http.md , but it isn't enought.
I don't know, whether it makes sense to update version. I use OSRM only for finding hiking trails on the single city.
I suggest you look at OSRM's changelog.
5.0 brings drastic change to the HTTP API, but also lots of improvements to how instructions are generated, as well as a lot of bug fixes and other tweaks. If you're planning on continuing to use OSRM, I would recommend upgrading.