SOCKS4 is not supported. And clipboard is probably not the correct format, just type in the SOCKS5 manually. Also make sure the proxy supports Minecraft which is usually not the case with free proxies or even most paid proxy provider. If you look for a good proxy provider, use our own service https://liquidproxy.net/
If it actually installed, it is located as you described at C:\Program Files\LiquidLauncher - if it's not, it's not.
Either try following this tutorial: https://liquidbounce.net/docs/get-started/installation
Or if it does not work, you can install LiquidBounce manually: https://liquidbounce.net/docs/get-started/manual-installation
Yes the currently script API can be sometimes very annoying to work with, like the calling a api that accepts a float with a number in js, etc. and the error usually is not very straight forward and needs a lot of experience to understand what's going on. we are still trying to improve it. At least you have https://liquidbounce.net/docs/script-api/debugging now, although you cannot create any named variable in the console. btw the docs looks much better than before.