You can find the specific commit here:
https://tvheadend.org/projects/tvheadend/repository/revision?utf8=%E2%9C%93&rev=2865&commit=OK
Broadly:
1. Clone the tvh repository
2. Switch to the release/4.0 branch (or stay on master, I doubt the code base is different that far back in time)
3. Checkout that specific commit as above
4. Build
The 1539 is the build number, yes. It's recently changed in terms of how it's calculated, but it's still basically a revision to the version number (e.g. number of commits in that series).