Question: Will an Apple Watch 4 work/pair with an iPhone 6 that has IOS 12.4. but IS NOT an activated phone?
The answer is...yes, but you need to activate it first.

From a hardware perspective, if you have an Apple Watch 4 with watchOS 5 and an iPhone 6 running 12.x, you meet the bare minimum of requirements.

The short answer is you need a SIM card to activate and upgrade the phone.  However, you don't need cellular service (well, maybe for a month).  If your phone is "free and clear" meaning you don't have a carrier obligation on it, you are free to take it to whatever carrier you want.

In this case, to activate your iPhone, it needs to communicate over the cellular network once.  After it's been activated you can disconnect your cellular plan and use it as an iPod Touch.  Everything on with iPhone with the exception of both cellular and WiFi calling will be functional.  In other words, you can use it to browse the web, but you can't make calls (including Skype).  However as long as you have WiFi, the rest of the functionality that requires data will work; including your Apple Watch.

Can you do this without activating?

Sure.  If the person giving you the phone doesn't restore it to factory defaults and you don't upgrade it at all, you possibly can use it without activating it but this seems like more trouble than it's worth.  You basically need a SIM to operate the phone.