If you are looking for information, as far as documentation goes check https://openwrt.org/docs/guide-user/network/vlan/start, there are several topics you should check for your use-case, especially VLAN Switch for VLAN access and traffic control.
Can OpenWRT is not allow NAT access for VLAN4 client machines also.
I still have two issues:
From a VLAN1 client machine, I can still ping 192.168.4.1, which is the vlan4 IP address of OpenWRT (it is not a big deal, but if possible, I want to hide that also.)
From a VLAN4 client machine, the Default GW giving by DHCP@OpenWRT is 192.168.4.1 . How can I change that to 192.168.4.2?
If you allow INPUT in the zone of vlan1 then 192.168.4.1 will reply, as it still is the router itself. You'd have to create a specific DROP/REJECT rule to block it in the INPUT chain.
You can add a custom option in DHCP server for interface vlan4 . Check the description of the first line here.
If you still have problem, you need to run a tcpdump to examine the contents of the DHCP offer, to make sure that options are applied and advertised properly.