Time on overview page 4 hours ahead (logs 4 hours ahead)

What does your config file say about your timezone config?

One possibility for system/system is browser cache and javascript. Maybe the changed info does not get applied and shown, if browser has old system page content in cache.

config system
        option ttylogin '0'
        option log_size '64'
        option urandom_seed '0'
        option hostname 'FB4040'
        option log_proto 'udp'
        option conloglevel '8'
        option cronloglevel '5'
        option timezone '<+07>-7'
        option zonename 'Asia/Bangkok'
  1. Logout / Login -> same as above
  2. CTRL-F5 -> same as above
  3. Close browser completely, start browser again -> same as above
  4. install zoneinfo-core zoneinfo-asia -> same as above

I think that the "local time" comes from browser's javascript, reflecting the local time in the PC/phone/whatever. And that is the time to be used by the "sync with browser" button.

Negative.
10) Changed PC time to 13:40, opened a fresh browser -> same as above (Luci shows now 20:40)

EDIT: I stand corrected. LuCI / System / System shows now indeed 13:46

what is the output of cat /tmp/localtime /tmp/TZ ?