why does my iphone randomly lock with a security lockdown for 4 hours

Your data is not lost if you have a backup either on iCloud or iTunes.

If you see an iPhone or iPad unavailable message or "Security Lockout" screen

If you forgot your passcode, you can use your Apple ID and password to erase and reset your device without a computer.

Learn what to do if you see "[Device] unavailable" or "Security Lockout" on your iPhone or iPad.)

Get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.

Read this Apple article -> About Lockdown Mode -> About Lockdown Mode - Apple Support

Not sure why you turn the Lockdown Mode ON.

What is Lockdown Mode?

Lockdown Mode is an optional, extreme protection that’s designed for the very few individuals who, because of who they are or what they do, might be personally targeted by some of the most sophisticated digital threats. Most people are never targeted by attacks of this nature.

When Lockdown Mode is enabled, your device won’t function like it typically does. To reduce the attack surface that potentially could be exploited by highly targeted mercenary spyware, certain apps, websites, and features are strictly limited for security and some experiences might not be available at all.

Maybe the password was “purse-dialed” incorrectly too many times.