Add: Announcement post for 15.0-RC4

[2TallTyler]

Long social message:

Third, er, Fourth try's the charm? You found some bugs that needed fixing before the full release of OpenTTD 15. Let's do one last (we hope) Release Candidate.

We've fixed two crashes related to station windows and signs, a hang related to implicit orders, and have stopped cacti from dying off completely with some tree spread settings.

Help us test our work!
Full news: https://www.openttd.org/news/2025/12/24/openttd-15-0-rc4

Short social message:

OpenTTD 15.0-RC4 now available on Steam / our website.
Third, er, Fourth try's the charm?

https://www.openttd.org/news/2025/12/24/openttd-15-0-rc4

Image:

News-15.0-rc4.zip