If you are trying to flash official lineage os rom or root package on you device via Twrp but if it showing error “Update error signal 4” then you can try following steps to fix this error.
There can various problem that can give Update error signal 4while installing lineage os rom.
How to fix Update error signal 4 while installing LineageOS.
The most common reasons for this error are:
1.Try to install Latest version of TWRP recovery on your device You can download the latest version from official TWRP Recovery website.
2.You are trying to install a build for a different device. You need to make sure you download the zip for the correct device *and* variant
3.You are attempting to migrate from an unofficial build to LineageOS. To do this, you need to flash the migration zip, marked as ‘experimental’
4.Your vendor/modem/bootloader is too old (or maybe too new). Flash the correct stock image for your device, before wiping data and attempting to install LineageOS again. This information should be listed on the device’s wiki page.
See also: Download Gapps For Lineage OS All Version 6.0, 7.1