Restriction is phone number country code based, can be bypassed. Also looks like its developers left setting to disable country checks at all. To do that, you just need to change android setting 'pixel.oslo.allowed_override' to '1' or 'true'.
Connect Pixel 4 with developer mode and execute command in adb shell:
adb shell setprop pixel.oslo.allowed_override true
adb shell "setprop persist.pixel.oslo.allowed_override true; setprop ctl.restart zygote"
You need root to do this.
Also there is Xposed Module in xposed manager to do this automatically called "EnableSoliOnPixel4", here's thread about it https://forum.xda-developers.com/pix...china-t3994917