I am plumbing in my new propane tank and decided to pull the copper
tubing as well and replace it.

In the future I'm planning to add a Propex heater (not sure where yet)
and possibly a second propane tank.

So, I'm thinking that I should put in a 4-way now and cap unused ways so
I don't have to break these lines again in the future.

Good plan?

Also, why do they run 2 small lines from tank to fridge & stove rather
than one line up and T it off inside?  I assume you'd need a bigger line
up to the T to handle the greater volume and "off to the races".  Having
not dealt with natural gases in the past, I'm wondering what the design
reasoning is here.

Happy Trails,
Scott