One more warning to disable [kde-next] should anyone still have it enabled.

The toolchain will move to GCC 5, this means a full rebuild of all in core, plus another 600 in main and apps (including a complete rebuild of all Qt & KDE)
Once core is rebuild, kde-next will start to be in use for further rebuilds. DO NOT use kde-next until further notice here. It can only be back in use once the above mentioned rebuilds are done.

It's a question, is there any reason to use 5.4 over 4.9 or over 6.x or over clang?

4.9 is starting to be incompatible with the latest some devs are expecting, so far they have been willing to adjust back for 4.9, but just a matter of time that won't be the case.
6 is too soon, waited here with 5 until most upstrean was ready for it, not needing hundreds of patches to get all to build, plus dealing with regressions not fixed upstream yet. Since Debian and Gentoo are not working with 6, 5 is the best fit for KaOS too.
Clang is used for many apps, no reason to fully switch or just set to one compiler

Rebuilds are now far enough along that it is possible again to boot into a working desktop with packages from kde-next.
Do not use on a production system, many rebuilds still to go and not tested on a lot of systems yet.

Full core rebuild, X-stack, sound stack, Qt 5, all of KDE (and a few hundred more) are now all rebuild/updated.
There are no more issues to boot into a working desktop, so this whole gcc5 build has now moved out of kde-next and is up in the build repo.
Build is approaching 1000 packages, once all is done expect some 1200 new builds for this gcc 5 move.

Testing will also include the creation of some minimal test ISOs.

No package will be updated on gcc 4.9 anymore, so don't expect the regular quick updates of applications. Everything will stay in build until gcc 5 is ready to move to stable.

I got new laptop, but i'm waiting for new iso with gcc 5.4, when it is expected?

Minimal internal test ISO will be up in a few days. Full stable ISO will be later this month or early August.