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I have an issue with WordPress Plugin for Instagram.

When I use IPv6 it said:

Instagram did not return a 200

But if I switch to IPv4 it worked correctly.

So Is there a way to keep ipv6 but request Instagram via IPv4.

Not a lot of info regarding your network settings to go on here.
It sounds like your wordpress server has internet access via IPv4. Do you have a way to confirm internet access from that server via IPv6? If you don't have command-line access or knowledge of the network the server's running on, it will be difficult.

Things to check in general would be:
name resolution: nslookup www.instagram.com (or whatever Fully Qualified Domain Name the plugin is using) - does it come back with an IPv6 address (AAAA) record as well as an IPv4 (A) record? (it probably does if turning on IPv6 breaks it for you)

Can you use "ping -6 www.instagram.com" from that host? The -6 forces IPv6 pinging - if that doesn't work, you may not have working internet access via IPv6

Some other IPv6 network setup checking you could do:

ip address

look for inet6 addresses that DON'T start with fe80: - those are link-local addresses that don't get routed past a local lan/vlan/layer2 broadcast domain network. If you only have FE80: addresses, you likely don't have IPv6 internet access.

Even if you do have non-link-local IPv6 addresses, it's possible you only have IPv6 connectivity to a local network, and not IPv6 internet access. If this is the case, and there's a default route for all IPv6 addresses to a gateway that only actually offers connectivity to local IPv6 devices, that would break an instagram plug-in, since Instagram does publish IPv6 addresses in the public DNS, and your system would have a route it thinks it can use to reach that public IPv6 address.

route -6

or

ip -6 route

Look for a default route, one with a Destination of [::]/0

Default routes CAN point to an FE80: address - some gateways will advertise themselves with their link-local address.

If you have working IPv6 on a private network, but DON'T have IPv6 connectivity to the internet, you might have to remove any IPv6 default routes and add back routes to only the IPv6 networks you can reach via IPv6.

An example using IPv6 addresses in the documentation IP range of adding a route to a specific IPv6 address through a gateway reachable on an network interface called eth0 is:

ip -6 route add 2001:DB8:0:2::/64 via 2001:DB8:0:1::1 dev eth0

You'd have to use an appropriate "ip -6 route del " to remove the default route as well.

Keep in mind adding and removing routes will likely require root or sudo access.

If you have reasons to leave your IPv6 settings alone, but can edit /etc/hosts you could create an IPV4-only entry in /etc/hosts for the instagram hostname. This is a quick and simple fix, but has the disadvantage that you need to monitor the public IP instagram publishes in DNS and update your hosts entry if they ever change it or the plugin will break again.

Not sure if this even comes close without more information on your setup, but hope it at least points you in some helpful directions.