Z370 Pro4 BIOS update fail

The first (and definitely the last) time I used the Windows update.

I have contacted the seller, but it turned out that the mobo has a seller warranty (24 months) and they do not respect the manufacturer's warranty (36 months). Looks like I need to buy a new mobo :(

Yes, that helps:

The following are AMI BIOS Beep Codes that can occur.

Beep Code     Descriptions

1 short             Memory refresh timer error.
2 short             Parity error.
3 short             Main memory read / write test error.
4 short             System timer failure.
5 short             Process error.
6 short             Keyboard controller BAT test error.
7 short             General exception error.
8 short             Display memory error.
9 short             ROM BIOS checksum error.
10 short        CMOS shutdown Read/Write error.
11 short        Cache Memory bad.

So based on the codes you are getting it indicates a RAM issue (or a bad flash that
effects the RAM).

Follow the instructions here to fully clear CMOS:
http://forum.asrock.com/forum_posts.asp?TID=630&title=how-to-clear-cmos-via-battery-removal

Then, when done, power the system on and leave it on for at least 10 mins. Let
us know if this gets you up and running again. If not, you will need to contact
ASRock's Tech Support department and inform them that you have had a bad BIOS
flash and your board is no longer operational. They will talk you through what you
need to do.

I don't understand what you mean by 2-4-3-2-4-3.
oo-oooo-ooo with o representing a single beep and - representing a pause?
Could you record the beep code and upload it to youtube so we can hear it?

Did the system say "Update Successful" when the update finished?
Did the update actually complete or did the flash get interrupted somehow?

If the flash did not complete correctly then you will need to contact ASRock
Tech Support:

https://event.asrock.com/tsd.asp